In the surah 17. Al Isra, verse 110, we find that Allah has The Best Names but didn't explain about how many names we should know. The ayat is "Say (O Muhammad ), "Invoke Allah or invoke the Most Gracious (Allah), by whatever name you invoke Him (it is the same), for to Him belong the Best Names. And offer you Salat (prayer) neither aloud nor in a low voice, but follow a way between."
Here are some hadits from Bukhari and Muslim explained about the number of 99 Asmaul Husna :
Bukhari, Book 8, Volume 75, Hadith 419
Narrated Abu Huraira:
Allah has ninety-nine Names, i.e., one hundred minus one, and whoever believes in their meanings and acts accordingly, will enter Paradise; and Allah is Witr (one) and loves 'the Witr' (i.e., odd numbers).
Bukhari, Book 9, Volume 93, Hadith 489
Narrated Abu Huraira:
Allah's Apostle said, "Allah has ninety-nine Names, one-hundred less one; and he who memorized them all by heart will enter Paradise." To count something means to know it by heart.
Bukhari, Book 3, Volume 50, Hadith 894
Narrated Abu Huraira:
Allah's Apostle said, "Allah has ninety-nine names, i.e. one-hundred minus one, and whoever knows them will go to Paradise."
Muslim, Book 35 : Hadith 6476
Abu Huraira reported Allah's Apostle (may peace be upon him) as saying: Verily, there are ninety-nine names for Allah, i. e. hundred excepting one. He who enumerates them would get into Paradise. And Hammam has made this addition on the authority of Abu Huraira who reported it from Allah's Apostle (may peace be upon him) that he said:" He is Odd (one) and loves odd number."
Muslim, Book 35 : Hadith 6475
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: There are ninety-nine names of Allah; he who commits them to memory would get into Paradise. Verily, Allah is Odd (He is one, and it is an odd number) and He loves odd number. And in the narration of Ibn 'Umar (the words are):" He who enumerated them."
