It’s sort of like asking what is the military strength of U.S. ground forces / infantry?
Is it just active duty Army and Marines? Add in Reserves and National Guard? Add in anyone who is a former member of same? Add in anyone in the unorganized militia who owns an AR or AK or similar, and has run a few hundred rounds through it?
I suppose you also have to ask who the adversary is in order to gauge the effectiveness of troop strength. Our “force” against ISIL “force” would not be the same as Jordanian “force” vs. ISIL force.
Numbers matter, however, and who in Iraq and Syria has joined them? How many are moving across the border to join them? What is their recruitment base - take that number, multiply by two and respond accordingly?
I’ve been reading a bit about Spartacus and part of the problem the Romans faced was they kept underestimating his numbers due to the fact that his army kept growing exponentially and unpredictably as he passed through regions and slaves joined his army. It’s a major reason he was so successful. That’s why Crassus did an overkill.