User blog comment:Legionwrex/I figured out how many Reapers there are!/@comment-72.89.53.204-20120803151122/@comment-2250460-20120803175319

18,000 is the minimum of Sovereign-Class Reapers, there are probably more, and there are definitely more destroyers.

So to answer your question, there are more than 18,000 sentient lifeforms overall.