Three, actually. Lay one down, wait for CD, trap one, lay another and have that go off, then finally pop Readiness and lay a third. Three mobs trapped. And absolutely no practical application whatsoever, because that will make you unable to chaintrap the first target.
Doubletrap however, can be very viable in instances, for difficult pulls. You trap two to begin with, burn down a mob, and keep chaintrapping your first target while the second is being dealt with.