Tours Ending?

AP saying:

Bush said 5,700 U.S. forces would be home by Christmas and that four brigades — at least 21,500 troops — would return by July, along with an undetermined number of support forces. Now at its highest level of the war, the U.S. troop strength stands at 168,000.

I am just curious, of the 5,700 soldiers, how many are due to come home because they have done numerous tours?

Of the 21,500 soldiers, how many are due home after a 15 month tour?