Im probobly wrong now, but I once calculated this out...
just for fun try it yourself if you have the nids codex
Lets play the game:
HOW MANY RIPPERS CAN I HAVE IN AN ARMY LIST?
I believe the number should (technically) come to around 180 rippers
If there was a cheap ripper sprue someone should attempt that... I might be wrong though, you might be able to get more

I know I would!!!