And Shaykh Ibn ‘Uthaymeen was asked about that and replied:

The names of Allaah are not limited to a certain number. The evidence for that is the words of the Prophet (peace and blessings of Allaah be upon him) in the saheeh hadeeth: “O Allaah, I am Your slave, son of Your slave… I ask You by every name belonging to You which You have named 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.”

What Allaah has preserved in the knowledge of the unseen with Him cannot be known and what is not known is unlimited.

With regard to the words of the Prophet (peace and blessings of Allaah be upon him), “Allaah has ninety-nine names, one hundred less one. Whoever learns them will enter Paradise,” this does not mean that He does not have any names apart from these, rather it means that whoever learns these ninety-nine of His names will enter Paradise. This is like when the Arabs say: “I have one hundred horses which I have prepared for jihad for the sake of Allaah,” which does not mean that the speaker has only these hundred horses, rather these hundred are prepared for this purpose.

Majmoo’ Fataawa Ibn ‘Uthaymeen, 1/122.