I was going to do 3x Acolytes but since we know they can come back even after being eradicated, I think 2x will be good enough for them. I'll probably do 3x Crabs/Tanukis.
There really isn't a hard number to apply to all commons though. You're better off making and testing army lists to figure out what you need and go from there.
3 sets is a good number for a lot of them in general, but there are some squads that are notoriously not strong enough to build an entire army around, like Monks, Deathstalkers, Tagawa Archers, etc. and those may function best as 2 squads supporting the rest of the list.
Then there are ones that demand more than 3, like Marro Drones, Zombies, Arrow Gruts, Venocs, Dividers, etc.