There is nothing wrong with saying “O Allah, send blessings upon Muhammad as much as the number of Your creation”

We do not see anything wrong with sending blessings upon
the Prophet (blessings and peace of Allah be upon him) in the wording
mentioned in the question. Sending blessings upon the Prophet (blessings
and peace of Allah be upon him) is a kind of du‘aa’ (supplication), and
when it comes to du‘aa’, some du‘aa’s are limited to specific formats
and wording, and some are unrestricted and the door is open to whatever
meaning the one who is asking, calling upon Allah and praying intends,
on condition that that does not include anything that is contrary to
Islamic teachings. 

Al-Haafiz Ibn Hajar (may Allah have mercy on him) said: 

Here we see Imam ash-Shaafa‘i (may Allah elevate his status)
– who is one of those who showed the most veneration towards the Prophet
(blessings and peace of Allah be upon him) – saying in the introduction to
his book, which is the primary reference for the followers of his madhhab: O
Allah, send blessings upon Muhammad… And towards the end he said: Every time
mindful people remember him, and every time heedless people forget him. It
is as if he extrapolated that from the saheeh hadith in which it says:
“Glory be to Allah the number of His creation.” Al-Qaadi ‘Iyyaad wrote a
chapter in his book ash-Shifa’ describing ways of sending blessings
upon the Prophet (blessings and peace of Allah be upon him), in which he
quoted marfoo‘ reports from a number of the Sahaabah and Taabi‘een. End
quote. 

This was quoted by Muhammad ibn Muhammad al-Gharaabeeli (835
AH), who was a close companion of Ibn Hajar, as it says in one of the
manuscripts that were found by Shaykh al-Albaani. See: Sifat Salaat an-Nabi
sall Allahu ‘alayhi wa sallam (172). See also: Asl Sifat Salaat an-Nabi
sall Allahu ‘alayhi wa sallam (3/939). 

The scholars differed concerning one who says, “O Allah, send
blessings upon Muhammad the number of such and such”, as to whether he would
be rewarded as much as that number, or he would be given one great reward.
What was stated with certainty by Shaykh al-Islam Ibn Taymiyah (may Allah
have mercy on him) was that he will be given one reward, and he is not going
to get the same reward as someone who actually does that number of salawaat
(sending blessings upon the Prophet (blessings and peace of Allah be upon
him)), but this one reward will be greater than the reward of one who sends
blessings upon the Prophet (blessings and peace of Allah be upon him) in the
regular manner (without adding such phrases). 

He (may Allah have mercy on him) said: 

If it is said to a man: Say tasbeeh twice, or say tasbeeh
three times, or one hundred times; he must inevitably say Subhaan Allah,
Subhaan Allah (Glory be to Allah, glory be to Allah), until he has
completed the number. If he wanted to make it brief, he would say:
Subhaan Allah twice, or one hundred times, but in fact he would only
have said tasbeeh once. The Prophet (blessings and peace of Allah be upon
him) said to the Mother of the Believers Juwayriyah,
“After I left you
I said four words which, if they were weighed against what you have said
today, they would outweigh it: ‘Subhan Allah ‘adada khalqihi; subhaan
Allah zinata ‘arshihi, subhaan Allah rida nafsihi, subhaan Allah midaada
kalimatihi (Glory be to Allah, as much as the number of His creation;
glory be to Allah as much as the weight of His Throne; glory be to Allah as
much as pleases Him; glory be to Allah as much as the ink of His
words).” Narrated by Muslim in his Saheeh. What is meant is that
Allah, may He be glorified, deserves glorification as much as that number.
This is like the hadith in which the Prophet (blessings and peace of Allah
be upon him) said: “Rabbana laka al-hamd mil’ al-samawaati wa mil’ al-ard
wa mil’a ma baynahuma wa mil’a ma shi’ta min shay’in ba’d (Our Lord, to
You be praise filling the heavens, filling the earth, filling everything in
between and filling whatever else You will besides that).”  It does not mean
that he said tasbeeh as much as that, because the phrase “as much as”
sometimes describes how much a person did, and what he did is limited and is
not infinite; and sometimes it means “as much as the Lord deserves”, and
this is a statement that deserves great reward. Otherwise, if the worshipper
says in his prayer, “Subhaan Allah ‘adada khalqihi (Glory be to Allah
as much as the number of His creation),” he has only said tasbeeh (glorified
Allah) once. When the Prophet (blessings and peace of Allah be upon him)
prescribed that the worshipper should glorify Allah (tasbeeh) thirty-three
times after each prayer, and praise Him (tahmeed) thirty-three times, and
magnify Him (takbeer) thirty-three times, if he said, “Glory be to Allah and
praise be to Allah and Allah is Most Great as much as the number of His
creation,” he would only have glorified Him once. 

End quote from Majmoo‘ al-Fataawa
(23/11-12) 

He also (may Allah have mercy on him)
said: 

The best of this type of supplication is that which is most
comprehensive in praise, such as saying,
“Subhaan Allah
‘adada khalqihi (Glory be to Allah as much as the number of His
creation)”. This is better than merely saying, “Subhaan Allah (Glory
be to Allah)”. And saying, “Al-hamdu Lillah ‘adada ma khalaqa fi’s-sama’, wa
‘adada ma khalaqa fi’l-ard, wa ‘adada ma baynahuma, wa ‘adada ma huwa
khaaliq (Praise be to Allah as much as the number of what He has created in
heaven, and as much as the number of what He has created on earth, and as
much as the number of what is between them, and as much as the number of
that which He is going to create) is better than simply saying, “Al-hamdu
Lillah (praise be to Allah).”

End quote from al-Waabil as-Sayyib min al-Kalim at-Tayyib
(p. 87) 

Ibn al-Qayyim (may Allah have mercy on him) said: 

The superiority of saying “Subhaan
Allah wa bi hamdihi ‘adada khalqihi wa rida nafsihi wa zinat ‘arshihi wa
midaada kalimatihi
(Glory and praise be to Allah, as much as the number of His creation, as
much as pleases Him, as much as the weight of His Throne and as much as the
ink of His words)” is
many times greater than merely saying “Subhaan Allah (glory be to
Allah).” That is because when a person says that, the meaning he will
reflect upon – “Subhaan
Allah wa bi hamdihi ‘adada khalqihi (Glory
and praise be to Allah, as much as the number of His creation)”
– learning about His attributes, declaring Him to be above all shortcomings
and venerating Him in terms of the number he mentions – is greater than if
he simply says “Subhaan Allah.” 

This is called multiple dhikr, which is greater in praise
than single dhikr. Therefore it is better than it. This only becomes clear
when you reflect on the meaning of this dhikr and understand it. When a
person says, “Subhaan
Allah wa bi hamdihi ‘adada khalqihi (Glory
and praise be to Allah, as much as the number of His creation)”,
this is a statement of what the Lord deserves of glorification as much as
the number of all created beings that ever existed or will ever exist, to an
infinite degree. 

This is a statement that declares Allah to be above all
shortcomings, glorifying Him and praising Him that great number of times,
that no one could count or enumerate. It does not mean that the one who says
this has said tasbeeh this number of times; rather he is saying that what
Allah, may He be glorified and exalted, deserves of tasbeeh (glorification)
is a tasbeeh that reaches this number, which, if there was a number greater
than it, he would have mentioned it. 

End quote from al-Manaar al-Muneef (p. 34) 

The author of al-Mawaahib al-Jaleel (1/20) quoted the
faqeeh Ibn ‘Arafah as saying:

He will be given a reward greater than that of one who sends
one salutation (upon the Prophet (blessings and peace of Allah be upon
him)), not the reward of the one who sends that number of salutations. End
quote. 

To sum up: there is nothing wrong with sending salutations
upon the Prophet (blessings and peace of Allah be upon him) with this
wording: “O Allah, send blessings and peace upon Muhammad, as much as the
number of Your creation, as much as pleases You, as much as the weight of
Your Throne, as much as the ink of Your words”, speaking directly to Allah
and asking of Him, because saying “O Allah (Allahumma)” is a call,
directly addressing Allah. One may also say, “May Allah send blessings upon
our Prophet Muhammad as much as the number of His creation…”. This is a
valid wording in linguistic terms. 

In question no. 126934 we stated
that this wording is mustahabb in the case of tasbeeh (glorifying Allah) and
tahleel (saying Laa ilaaha ill-Allah), and there is no difference between
that and sending salutations upon the Prophet (blessings and peace of Allah
be upon him), because these are all adhkaar that are prescribed. We hope
that Allah, may He be glorified and exalted, will decree a reward for this
salutation greater and more abundant than the reward for a regular
salutation that does not include these phrases, but that is subject to
several conditions, as follows: 

1.
the one who says it should not believe that it has a
special virtue

2.
he should not think that it is a confirmed Sunnah from
the Prophet (blessings and peace of Allah be upon him)

3.
he should not believe that it is better than the wording
of as-Salaah al-Ibraaheemiyyah that was taught to us by the Prophet
(blessings and peace of Allah be upon him). This has been explained in fatwa
no. 88109

4.
he should not use it in the prayer; rather it should only
be used outside of the prayer, because the prayer – whether obligatory or
supererogatory – has its own adhkaar that are proven from the Prophet
(blessings and peace of Allah be upon him).

And Allah knows best.