It is tough for a team that only has one team and not a lot of players to pull from to make it deep in big tournaments. You do not want to take players and not play them but you also want to play your best. There is the never ending question in older teams, Am I going to showcase players or win the tournament? Very rarely can a smaller team go in with both mentalities. The bigger organizations not only have the better players, per average, but they also have a bigger pool to pull from. They can bring kids in from other teams in their organization and can bring kids up from other age groups. A team this summer won a big tournament in a big southern city, no names to protect the innocent, and their final four pitchers were not on the roster for the tournament. They were in their organization to some extent and were legal players for that age group but were not on the roster. A couple of them came in from another tournament that week in a different city. The average team could not do that.
I say all that to say you need to have enough to be competitive and not too many to be the guy who brings kids and doesn't play them. I also say you have to know your pitchers on your roster. My son can go out and throw the limit every time and a friend of mine's son can only throw about 50 pitches. I know another kid who played with son this year that was only good for 25 pitches at the most and everything went away, speed and control. So if you have my son on your roster he could throw a complete game every weekend, (except he is on pitch count this fall due to number of innings thrown spring and summer). Most of the tournaments in the fall usually have small bracket play due to time constraints so make the pool play games count.
All of that leads to there is no set number. Depends on your pitchers and your goal.