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Amtgard 6.0 (don't know about anything newer) doesn't do anything so silly as counting layers of foam. The rules are that you can't feel the core through the padding, no point on the weapon can be small enough to poke someone in the eye (6.1 I think set this to 2.5" minimum?), striking areas have to be covered in cloth (exposed tape stings), and OFFICIALLY red weapons have to have at least 18" of blade length, at least 3' total length, and weigh at least one pound per foot of blade length. (I've never seen a red weapon be disqualified for being too light, though.) The major difference by not counting layers of foam means that it's legal to use fewer layers of thicker foam -- in fact pool noodles are a common construction material because they're the right diameter, have a hole down the middle for putting a core in, and are fairly inexpensive and lightweight. (Not to mention generally easy to replace when the weapon wears out.)

Of course, the main rule of thumb is that you have to let someone else hit you with it. If you wouldn't want to be hit with it (or if it hurts you when you're hit with it) you shouldn't be hitting others with it.

Edit: This does mean that it was legal for my 6' staff to have 18" of striking area on either end and just wrapped tape down the 3' in between. I did cause a few sore knuckles using that staff from people blocking with their weapon hand, which is probably the kind of thing that prompted the padding rule, but I still think it's absurd.