I am coaching youth girls (11-12) and we currently have 13 on our team. This is not enough for two teams, however, we may have more girls join due to some transfers. I was told we will still have one team. Has anyone else been in this situation? Some girls are in other activities so we will not always have 13 at a time, but we will usually have more than 10. I want to have equal playing time as I feel that is the best way for them to learn.

Any advice you can give me on substitutions and playing time?

I think that 8-9 players is just about right. Otherwise, like you said, there is too much sitting on the bench and not enough meaningful minutes.

You could make do with what you have or you could rotate the rosters. For my youth team this year, we had 12, and I was planning to tell parents that I want to have 8-9 players at game and tournaments. Otherwise there is too much sitting on the bench. So I was going to ask 2-3 players to stay home for each game. Then rotate those players that sit out so it's even opportunity.

However we had one player move and another decide not to play basketball anymore. So we are down to 10 and it's not as much of an issue. I still might ask one or two players to stay home but it might not be necessary since we'll usually have at least one player with a conflict, sick, or something.

Anyway, I don't know if it's a good way to handle things, but that's the plan for now.

As far as subbing players, I attached a couple excel template you can use to fill in the blanks and give players almost exactly even playing time. And then it's the coaches choice at the end of the game. However it's not perfect because someone always gets a little extra time depending on how many sessions you use.

I have also found that for younger players, subbing 3-5 players at once was chaotic. They'd forget who they were guarding, etc. I found that subbing 2 players at a time and requiring them to hand out towels worked much better:
http://jeffhaefner.com/coach/youth-defense-how-to-make-sure-young-players-know-who-they-are-guarding/

If you use a numbered break, it also helped to keep players in the same positions. They'd forget, now I'm the 4 and I need to throw the ball in bounds. I tried not to change that on subs.

Ok, the attachment isn't working. I uploaded it here:
http://breakthroughbasketball.com/dl/playerrotation.zip