ISIL (al Qaeda in Iraq and the Levant) is believed to have about 15,000 personnel in Iraq and Syria. Most are armed, although many are not trained fighters and tend to work in support jobs. It is estimated that some 30,000 people have come to join ISIL since early 2014 but over half have since been killed, deserted or (more rarely) were not accepted in the first place.
Most of these volunteers came from Moslem countries with the largest source being Saudi Arabia. Moslems living in Europe comprised about 15 percent of the volunteers and about 70 percent of those came from France, Germany, Belgium and Britain. Very few (2-3 percent) came from the United States.
All this is expensive. ISIL incurs costs for every volunteer it accepts. Those with skills are paid a monthly salary, often more than they could make back home. Even the unskilled, if accepted, cost ISIL cash for food, weapons and other necessities. Suicide bombers and unskilled fighters are paid no, or little, cash but cost money to give them a minimal amount of training. Those who volunteer for suicide bombing require the services of several highly skilled bomb makers, planners and handlers. Those assigned to infantry or security duties require a few weeks training on how to handle their weapons, some basic tactics and some stressful exercises to weed out those who cannot be relied on to be dependable in combat. Many ISIL volunteers are damaged goods. Those who are mentally unstable or delusional are not wanted, even for suicide bombing.
ISIL is currently suffering a cash shortage. This is the result of more air strikes that ignore the use of human shields. For over a year human shields were effective because the Americans were obsessed with avoiding any civilian casualties even if it meant they had to abort most attacks on very visible ISIL economic targets. That changed after the November 13th attacks in Paris. The U.S. was already under pressure to loosen up their ROE (Rules of Engagement) because since October the Russians were taking out many of the targets the Americans avoided because of ISIL human shields. ISIL is still attracting a lot of volunteers but the quality is declining. Many of the more skilled potential volunteers are having second thoughts about the future of ISIL.