a true hip hop classic album to me:
1. genre defining - if i was to introduce to a noob/non-hip hop head a list of albums that they must hear to spark their interest in the genre, these albums would make the list
2. influence - it has to have significant influence to future material by other artists, or change the genre in some kind of way
3. quality lyrics/beats throughout
for my first reason, i think this addresses the OP's point about the fact that there could only be around 20-30 true classics in the genre
any 'classics' beyond this 20-30 album list would be something i call 'personal classics', 'cult classics', 'regional classics', whatever.
for my second reason, since albums like 36 chambers, chronic, amerikkka's most, etc already defined the 'sound' of a particular / group of artists / label, the status of the other albums that follow such as ob4cl, liquid swords, doggystyle, death certificate are very debatable, especially when some of them have actually surpassed their predecessor. i would put these as 'second-tier' classics
realistically, everyone's list should have no more than 10 albums. say if you were to explore to a new genre, you would only need 10 albums to form some kind of basic opinion about whether that genre is for your or not if you know what i mean