Via Seatalk you can hook up as much as you like, that is I wouldn't hook up 50. That's a bit over the top.
Be advised there are two head-units looking very similar one carries all the electronics inside to drive the motor and also has the connections for compass and rudder sensor. The other one has only Seatalk and NMEA. This is the one you can use as additional one or is also used in conjunction with a separate course computer.