In years past, the biggest concern of coaches seemed to be in getting games/playing time. Therefore, pool play 0f 3 to 4 games was popular. Pool play wasn't and still isn't meaningless with pool play results being used for seeding into the single elim phase. Games were on a time limit and ITBs were used to break ties. Over the last 2 to 3 years, TDs have not played pool play games to full conclusion because of what you have discussed above, i.e., just how important is the pool play game if the winner is based on the elimination round.
From previous discussion on OFC, games/playing tie is still very important, but this year lots of tournaments are going to 2 pool play games followed by double elim, with longer time limits. This is going to mean the players will be as tired and maybe more tired than in previous years, especially if they make a run back up thru the loser's bracket.
I know this doesn't answer the question of how many players--- I hope it causes some discussion on just how much coaches and parents should expect of 10 and 12U players, especially when they are worn down. I don't know if it is statistically true, but it seems to me the more tired players are, the more susceptible they are to mental mistakes and errors and injuries. If you know what you are expecting, I think that will help toqwards deciding how many players.