Here, according to the U.S. Army's own data, is the disposition of its combat brigades:

16 deployed (13 in Iraq, 2 in Afghanistan, 1 in Korea)
20 in downtime between deployments
1 in transit to or from deployment
1 in global response force
1 in homeland defense/homeland security
1 at Fort Riley, training advisers
1 new brigade, being built up, estimated ready in 2010
2 for "usage factor" (see below)
------------
43 Total

Therefore, the number of additional brigades available for, say, reinforcements to Afghanistan = zero.

A word about this "usage factor." When a new brigade combat team arrives in Iraq or Afghanistan, the BCT it replaces stays in place for 40 days to conduct "battle handover." This 40-day period amounts to 0.11 percent of the deployment. So, the Army needs to deploy 1.11 BCTs for each BCT that's needed. So, the 15 BCTs really means (15 x 1.11) 16.65—or, to round up, 17 BCTs. Hence, the "usage factor" of two brigades.

One more note. To put 7,000 more combat troops (two BCTs' worth) into Afghanistan (or, for that matter, Iraq) actually requires more than 7,000 total troops. They need to be supported by what Army planners call "enablers"—military police, headquarters staff, an additional aviation brigade, etc. This is why the surge in Iraq, which President Bush and Secretary Gates thought would amount to 20,000 troops, actually ended up totaling 30,000. Similarly, 7,000 extra troops in Afghanistan would actually mean about 10,000.