The current roster as per the Toronto Marlies website contains 31 players. The roster is filled by players that are designated there by the Toronto Maple Leafs, players called up from ECHL affiliates, or by signing free agents (other ways exist as well). The Marlies along with the rest of the AHL, don't entirly have full control of their roster which at some points can be devastating.
For instance, if the Leafs suffer some injuries they may summon the services of a Marlie. This may come on a game day and regardless the Marlies have to deal with the loss. That can definitly drive a coach nuts. In most cases the Leafs have been respectful in terms of not putting the Marlies in complete jeopardy by calling up too many players or for other reasons.
The importance of depth is key, and getting everyone involved in the action is usually an easy task, although some players may not get as much as they'd like. The point is to develope the players and give them professional experience to help them grow. It's a tough job being a coach, but these men know what their doing and deserve tons of respect in my opinion.