Garmin Forerunner 305 cadence sensor add on 2?

I have the Edge 305 and have three bikes with three GSC-10's (speed/cadence), so yes, not a problem at all. You may need to synchronize when you switch bikes, but that takes 5 seconds to go through the menu until you get to accessories and cadence sync.

I really don't think there's a maximum number of sensors since you sync each one before the ride.

Yes, you can have any number of cadence sensors, but you need to resync each time you change bikes/sensors.

yes.  you can have as many sensors as you need.  That includes cadence, foot pods, hart rate monitors, what ever else they have.  You just have to do a new sync each time you change from one cadence to another, or one HR to another, ect.....