why was there only a limited amount of characters in Noh drama?

Answer 1

There is the practical and the theoretical.
The practical is that as traveling troups, there could only be a limited number actors (and costumes) to haul around. That limited the number of characters that could be on stage at one time.
The theoretical has to do with what kind of relationships are allowed in the dramas. If you look back at Greek drama, you will find that the number of characters increases only gradually. There is considerable debate at each step, protagonist, antagonist, etc., as to how many voices should be speaking with the chorus.