So far, I have come up with this:
Spoiler:Obviously, having 20 A_JumpifInventory's is a bit much, but I'm drawing a blank on how to do it in a simpler way without ACS. If it requires ACS to solve, I would like some simple advice, given I'm an ACS newb. :p
Edit:
Okay, Zippy mentions that you can use a >= operator in Decorate in the thread I linked. How do you do this? Is it as simple as A_JumpifInventory("Ammo",>=20,1)?
Yeah I'm a total newb. :|
Edit 2:
Did a quick test, and apperently it will jump if the value in A_JumpifInventory is greater than or equal to the amount specified. I didn't realize this. Disregard then. :)