Allah’s Names and Attributes are unlimited in number, even though what we know of His Attributes is more than His Names.
The Prophet (saw) said, “Allah has 99 names (100 minus 1), whoever memorizes (enumerates) them will enter Paradise” [Bukhari/Muslim].
This is the authentic version. The inauthentic version continues on to list all 99 names. This addition was added by the narrator Waleed bin Muslim, and his students thought it was a part of the hadith. The narration with this addition is found in Tirmidhi, and Tirmidhi himself after recording it ruled that this hadith was weak. And this grading is almost unanimous amongst the scholars of hadith.
What this hadith means, that out of Allah’s INFINITE names, there are 99 specifically which if you memorize them you will enter Paradise.
No scholar has ever said there are only 99, and no Sahabah ever asked, which 99 are they. The point is to learn as much about Allah(swt) as we possibly can. Going through just the Qur’an one can find Allah) has well over 99 Names. Another proof that Allah (swt) has an infinite number of Names is the following dua of the Prophet (saw), “O Allaah, I am Your servant, son of Your servant, son of Your maidservant, my forelock is in Your hand, Your command over me is forever executed and Your decree over me is just. I ask You by every name belonging to You which You name Yourself with, or revealed in Your Book, or You taught to any of Your creation, or You have preserved in the knowledge of the unseen with You, that You make the Qur’aan the life of my heart and the light of my breast, and a departure for my sorrow and a release for my anxiety.’”
We know that Allah has an unlimited number of Names. Of them, we have been told of a some of them. Of the ones we have been told about, 99 of them have this special significance attached to them. This does not mean that there are only 99 Names of Allah, nor does it mean that all the ones we know of fall into this special category.
It is simply that Allah has an unlimited number of Names, and of the ones He has informed us of, 99 of them hold the significance indicated in the hadith.