Allaah’s Names are not limited to any specific number, and this is understood from the statement of the Prophet (may Allaah raise his rank and grant him peace) in the well-known narration: ))
«…I ask You by every Name that You have, those You named yourself with, those You revealed in Your Book, those You taught one of Your created beings, and those You kept with Yourself as knowledge of the Unseen…»
It was collected by Ahmad, Ibn Hib-baan, and al-Haakim, and it is saheeh (authentic).
The Messenger (sallallaahu `alayhi wa sallam) classified the Names of Allaah into three categories from all the Names He Named Himself with:
 Those He has revealed in His Book,
e.g. bismillaahir-Rahmaanir-Raheem (In the Name of Allaah, ar-Rahmaan, ar-Raheem).
 Those He has taught one of His created beings
e.g. It is not in the Qur`aan, however, it has been taught by revelation to one of His Prophets. How can anyone be taught a Name of Allaah that is not in the Qur`aan unless he was a Prophet who received revelation? So, he is referring to the Names he learned about Allaah from the revelations that is not in the Qur`aan. An example of this would be al-Witr. Allaah is al-Witr (the Unique One). The Messenger (sallallaahu `alayhi wa sallam) said:
Allaah is Witr , and He loves Witr
So, reference to Allaah being Witr is not found in the Book of Allaah, rather, it comes from the Messenger of Allaah (sallallaahu `alayhi wa sallam).
 And those He has kept with Himself as knowledge of the Unseen.
e.g. We have no knowledge of this third category, because Allaah has chosen to keep some of these Names to Himself. So, we do not say, just because we know all the Names in the Qur`aan, and that we’ve gathered all the Names in the Sunnah, that we could have possibly gathered all of Allaah’s Names. We have only been given a little knowledge with regards to His Names. How many Names are there that He has kept for Himself and not revealed to anyone? We do not know. Knowledge of that has not been given to us. We only know the Names that He has Named Himself with in His Book, and the Names His Messenger (sallallaahu `alayhi wa sallam) has conveyed to us. Beyond that, we do not know. We do know from this reference of the Messenger (sallallaahu `alayhi wa sallam) that there are other Names that He did not reveal to us, and that His Messenger (sallallaahu `alayhi wa sallam) did not even know about.
That being the case, we have another proof against those who name Allaah with Names other than those in the Book and the Sunnah; those who go against the tawqeefiyyah nature of Allaah’s Names. They name Allaah with names like Vishnu, al-Muhandis and so on. They name Allaah with names that they feel are good in their meaning, but have no support in the Book or the Sunnah. This is not permissible based on this hadeeth. This hadeeth gives us three categories of Allaah’s Names, and one of them is not good names that you deduce from your own intellect. Rather (we have been given a classification of) Names that Allaah has revealed in His Book, Names that He has taught one of His Prophets, or Names that He has kept for Himself. So this is a refutation and a further support for what was mentioned about Allaah’s Names being tawqeefiyyah.
We have a clear indication from the Messenger (salallaahu `alayhi wa sallam) that Allaah has Names we do not know of. This being the case, anyone who can mention the ninety-nine Names, we say: ‘Those are from the Names that we have knowledge of and there are others we do not have knowledge of.’ In fact, many of the scholars of the past made attempts to enumerate the Names of Allaah so that they may attain the reward from Allaah mentioned in this hadeeth we will discuss. They gathered ninety-nine, and they kept gathering because they needed a principle by which to extract the Names from the verses that would land them on ninety-nine. Some of the scholars had lengthy, lengthy lists of Allaah’s Names that reached up to two hundred names while they were initially trying to gather the ninety-nine mentioned in the hadeeth. However, their principles led them to gather more. Some of them like Ibn Hazm could not count up to ninety-nine, they only got to eighty-some or seventy-some. So, they differed in the numbers of names they gathered, and they did not have the same number although they all tried to gather ninety-nine.
It is not possible for anyone to count or have complete knowledge of the Names that Allaah, the Exalted, has kept with Himself as knowledge of the Unseen.
 Collected by Ahmad in his Musnad (1/391, 452)
 Refer to Saheeh Ibn Hib-baan bi-Tarteeb Ibn Balbaan (972)
 Refer to al-Mustadrak (1929)
 Refer to Silsilatul-Ahaadeethis-Saheehah (199) of al-Albaanee and Muqbil ibn Haadee’s comments on Al-Mustadrak (1/696). For further study, review the criticism mentioned in Al`Ilal of ad-Daarqutnee (5/199-201) and Tahqeeq Musnad Ahmad (6/247-250).
Source : Allaah’s Names are not limited to any specific number – Al-Qawaa`id al-Muthlaa , Explained by Moosa Richardson, Notes by Umm Sufyan