There is one private organization that has been doing research on this subject and has stated a modest figure based just on known terrorists that are here at 20,000. They said that this figure naturally does not cover those that are sneaking in daily but rather known groups that are on the terrorists watch list.
Brother, I honestly think this is a conservative figure, but let's say that it's right. Assuming that we know where all of the are, can you imagine how many agents or law enforcement officers it would take to properly watch them 24/7? Even if you had sufficient manpower watching them, what's the chance of being able to stop all of them?
Now, I'm going to surprise you, and many might not believe me. If you include supporters and sympathizers, I would put the number over 100,000. I can guarantee you that they aren't being watched closely enough. We would need a fairly large army to watch them 3 shifts each day, 7 days a week. Sadly, many attacks are going to happen, and it's too late to stop them. The best we can hope for is to limit the destructive power of the weapons used against us, and that might be too late also.