There are 30,000-100,000 ISIS fighters, and they've been smuggling fighters in for long time now, and have gotten good at it. I made a promise that if things were ignored it'll get worse, the only change the numbers are going to get is an increase if the status quo continues.
Where are they specifically coming from? The 30.000 (which is the highest figure) are in Iraq/Syria, and despite the myth in some circles there is border security in both Canada and Mexico. I can't see any feasibility to the idea of ISIS fighters being shipped over as if it's D-Day.
Where are they getting reinforcements from?