20,000 since March 2003 works out to 555 a month. How many of the those are foreign fighters?
The question is, how many jihadists are coming in? I seriously doubt it's anywhere near 1000 a month that would be needed to sustain major operations.
It is my understanding that foreign fighters make up 4 to 10% of the 30,000 strong insurgency. The 'myth' of Iraq's foreign fighters
Further, places like Iran and Syria can't "suggest" too many of these people go off to be killed, because they need them in their own armies.
The numbers are relatively small compared to the size of the military, especially in Iran.
Still, whatever number you use, the fact that everyone seems to forget is that there is a FINITE NUMBER, and it is rapidly decreasing. Further, all previous wars/insurgencies show that with each additional terroist death, it becomes even more difficult to win.