2" Stack Vent

How many fixture units can a 2" stack vent serve. I have a 3" stack that serves a full bathroom on one side side of my house, a 1.5 inch stack that serves a kitchen and a 2" stack that serves one toilet and a sink. I am considering connecting my basement bathroom addition to the 2" stack (easiest to get to). How many more fixtures units could I add to the 2" stack?

I probably should clarify that the stack comes out of the basement floor as a 3" stack until just above the toilet discharge and reduces to a 2" line up to the sink and through the roof. Not sure it this matters or not.

The 2" vent will be adequate, but you have to attach your vent at least 42" above the upstair's bathroom floor.

Thank you!