Fasting only once per month

If you mean fasting once in every month as well as fasting the entire month of Ramadaan, there is nothing wrong with this and this deed will be rewarded.

A man came to the Messenger of Allaah (Peace & Blessings of Allaah be upon Him) asking him about Islam. The Messenger of Allaah told him to pray five times a day. The man asked, “Do I have to do any more than that?” The Prophet told him, “Only if you want to.” The Messenger of Allaah told him that he had to fast the month of Ramadaan. . The man asked, “Do I have to do any more than that?” The Prophet told him, “Only if you want to.” The Messenger of Allaah told him about zakaat. . The man asked, “Do I have to do any more than that?” The Prophet told him, “Only if you want to.” The man went away, saying, “By Allaah, I will do no more and no less than this.” The Messenger of Allaah said, “He will be successful, if he is telling the truth.” (Reported by al-Bukhaari, 2481).