Well that is the $1,000,000 question and there is no real answer. Some teams like to be really lean and have around 8 players. I really don't like that but it is obviously economically good. On the other hand I often see players with 21+ players, I can't see any point in this except as a very fleeting event. This is what I aim for.
I am doing 2 position training PG/SG so I like to have 6 trainees. 3 PGs and 3 SGs that I can train. My main dude at each spot, my backups and some young hopefuls I can train in the scrimmage.
I like to have 2 SF.
I like to have 5 big men. 2 starters, 2 backups and one BWS who can play when needed if someone is injured, plays too many minutes, play in cup games etc.
So that is 13 players. In addition I often have someone I am selling so that can bring me to 14. Actually at the moment I have 15 because I have one of my draftees that I just can't decide what to do with so I am not willing to sell him off cheaply or sack him.
Long answer, hope it is useful.