Why not 2 million more troops? Or 10 million?

"There are already more than 100,000 international troops in Afghanistan working with 200,000 Afghan security forces and police. It adds up to a 12-1 numerical advantage over Taliban rebels, but it hasn't led to anything close to victory. Now, the top U.S. and NATO commander in Afghanistan is asking for tens of thousands more troops to stem the escalating insurgency, raising the question of how many more troops it would take to succeed." (thanks Moustafa)