My guess would be something in the 200 to 500 range. Not that ISIS couldn't get several thousand across the border. Heck, thousands of other illegals come across every day. I just figure ISIS doesn't have that many fighters that are sophisticated enough to travel internationally and stay low-key in the US. By and large they seem to be fanatical {expletives}. If there were more than a few hundred ISIS {expletives} here, they'd probably be making their presence known. I think they are here, they're just not quite ready to go public - yet.
Maybe they're waiting until just before the elections to try to help out their buddy obama. Maybe they'll wait until 2016. Don't know. But I would bet anything you care to name there are at least a couple hundred ISIS types here already.
That's all they need to pull off a Beslan-style massacre. Most Americans have no clue of the terror that is going to unfold in this country.