User blog comment:Legionwrex/I figured out how many Reapers there are!/@comment-5337704-20120802054416/@comment-899316-20120802055129

I think that Prothean incompatibility took the Reapers by surprise. They had intended to make a Prothean Reaper, but they couldn't, so they settled for the next best thing, and kept all of the Prothean husks around as tools. That the Reapers could be taken by surprise by this indicates that it was the first time this had happened. At the very least, it's incredibly infrequent, with the inclusion of the Keepers tallying total rejections at a whopping two.