If you are playing 3 games every week, you may be getting too many minutes with some of your 10 players, and their game shape will suffer. There is also a risk of injury in scrimmages. So you probably want a few more than 10.
With 15, it may be difficult to keep all 15 in good game shape. And a backup with bad game shape and less skills will get pulled out of games, and your better players may still get too many minutes. You might stick with 15 for a couple of weeks, and if you see that a player is playing 0 or just a few minutes per week, sell of fire them.