Great question! I have tried many combinatins, and still play a few. I believe the more players you have, the more difficult it is to develop 'tight' music. It is true, that I think you acheive a real big advantage with thre quality singers (over two), and I think it true that three instruments begins to get you a real 'band' sound sound, if they are good players and they know the music.

So I would say three may be the optimum start of a good band. Personalities are also very important, and this is where the 'too many cooks' danger coems to play, so again, 3is probably the optimal start of a good band - add the fourth or fifth when the core is very settled, and all agree on the new member/role.

Having said that solo, is awfully easy (redo arrangements on the fly, not have to wait for someone to tune,or switch instruments, etc.). And a duo gives a good solo performer an additional 'foil' on stage (assuming there's good chemistry).

Guess you asked a question, we turned over a rock and found another question and two more rocks!