User blog comment:Legionwrex/I figured out how many Reapers there are!/@comment-4950455-20120801013113/@comment-2250460-20120801014111

While it is possible that there hasn't always been a 50,000 year return date, it is unlikely, the Catalyst has shown it doesn't like change that much.

It's more likely that the Reapers have taken heavy casualties the last couple of cycles.

Additionally, I don't remember any codex entry that says the Reapers don't have major numerical superiority, nor any cutscene that suggests otherwise.