As many as you can fit and afford!
If you keep wide open speed control time to a minimum you might cruise all day no problem.
Any solar offset to the motor draw has great effect on trip duration or distance. You dont HAVE to be able to 1 for 1 the thing, but if you can, even better.
4-8kW on a pontoon though...... 2kW is quite meaningful. Most of the pontoon boats i have entertained designing for could only fit 8 or so 300W modules.
