you also need to keep device type in mind.
Little alligators that shoot large bubble out of their mouths do not use a lot of air.
A bubble wall or large air stone uses a lot of air.
I have a Rena 300 on a 70 gal to run 2 large airstones. According to Rena, I should be able to run 8 devices off this pump. It seems that I could maybe add a 3rd and thats it (based on how much adjusting of valves I need to do to keep both stones flowing equally)
Hope this helps