The Names of Allaah are not confined to a fixed and definite number.
This is based on the famous hadeeth: "I ask You, O Allaah, by every one of Your Names by which You have named Yourself or revealed in Your Book. Or (those which You have) taught to one of Your creatures or appropriated for Yourself in the knowledge of the Ghaib (Unseen) that is with You." 
Specifying and grasping whatever Allaah has appropriated for Himself in the knowledge of the Ghaib (Unseen) that is with Him, is impossible to attain.
The way to combine between this hadeeth and the other authentic hadeeth: "Verily, to Allaah belong ninety-nine names, (by which) whosoever takes account of them (i.e. memorizes, learns and supplicates by them), will enter Paradise" is that the meaning of this (latter) hadeeth is:
"Verily, from among all the names of Allaah are ninety-nine names by which if someone takes account of them, he will enter Paradise."
It does not mean that Allaah's names are restricted to this number (of 99).
The equivalent of this would be if one were to say: "I have one hundred dollars which I have counted out for the purpose of giving in charity." This does not negate that he has other dollars in his possession, which he has counted out for a purpose other than charity.