Full Attendance Vs Oversubscribed - what do you do?

How do we get as many people to our sessions as possible without running the risk of having too many?

We have just the right amount of young people subscribed to our project - but of course due to the issues in their lives they cannot all attend all the time.

If we were to subscribe more young people we run the risk of them all turning up at the same time and leaving us over-subscribed.

If we don't subscribe more young people we run the risk of showing less than full attendance at our projects.

I'm sure there are plenty of organisations who have already dealt with this issue so does anyone have any advice or thoughts?