JWing---an interesting article from Haaretz on the tactics of the IS indicates with new recruits their strength is being estimated at the 20,000 range which as you indicated they are holding as their reserves in the rear---estimated hardcore fighters at 3,000.

This does not evidently include the Sunni coalition forces from IAI, al Duri, and the tribes.

http://www.haaretz.com/news/middle-east/1.606590