Journey,
I can't quote every source but believe currently we have 120,000 armed forces personnel in Iraq and about 1.4 million overall.
However, it gets tricky because some of the Armed Forces personnel in Iraq are reservists and National Guard which do not count against the 1.4 million overall total. Also, many military personnel typically have no reason to deploy in time of war. Examples: military personnel working in foreign embassies, recruiting duty, in the National Security Agency just to name a few.
So plain stats, say about 9 percent. However, it strains the military much more than that as it consumes a larger percentage of deployable personnel and resources. Sorry for convoluted answer.
Mud