How many people will a 20 ft pontoon boat hold?

This depends on the boat itself, length alone is not the determining factor. Every boat should have a specification plate on the right side of the transom (the rear board of the boat that the motor or sterndrive is attached to). This plate gives the model and serial number of the boat. It also included the maximum number of people or maximum weight of people and gear. It is important to respect the max weight for the stability of the boat at all speeds.