Conversation so far:
when did the library of alexandria burn down
The Library of Alexandria, or part of its collection, was accidentally burned by Julius Caesar during his civil war in 48 BC, but it's unclear how much was actually destroyed.
what really happened to the library of alexandria if it wasn't completely burned down
Despite the widespread modern belief that the Library of Alexandria was burned once and cataclysmically destroyed, the Library actually declined gradually over the course of several centuries.
when did the library of alexandria start its decline
The Library of Alexandria declined gradually over the course of several centuries, starting with the purging of intellectuals from Alexandria in 145 BC during the reign of Ptolemy VIII Physcon.
when did the library of alexandria disappear
Between 270 and 275 AD, the city of Alexandria saw a rebellion and an imperial counterattack that probably destroyed whatever remained of the Library of Alexandria, if it still existed.

Question: how many books did the library house
Self-contained rewrite: how many books did the library of alexandria house