It depends on a lot of things, but a couple of inches is pretty good. If you need to go thicker, you can add more assuming that the duct seal is in some kind of a box with a solid back. Shoot a shot, peel it up and see if it went through the ductseal. Shoot a few more in the same spot and check again. If it looks questionable, add some more. It's really dense though.
I do not shoot BBs at ductseal. The BBs will stick in the ductseal and if another BB hits one that is stuck in the ductseal, you will get a ricochet. Shoot lead pellets ONLY. They are soft, and if another pellet hits on a pellet already embedded in the ductseal, they just sort of mash together and there's no bounceback.
If you want to shoot BBs, you need a different type of trap.