So, I'm level 30 and I never really understood how you get more RZs.
This morning I bulldozed an omega RZ. It was just a construction site, never upgraded. I was expecting to get back an extra "RZ point" as you would with factories and shops, but that didn't happen.
EDIT: I was mistaken; I didn't bulldoze an omega RZ. The game doesn't let you do that. It was already upgraded once.