20 - always 20. Never more or less it's 20 ammo every single time.
Is there a way we could have these handled something like wands? Or is it too complex? I'm sure you can split a stack of ammunition (or merge it with another pile) whereas wands won't let you. Is this why this hasn't been done? Wands also have a much more finite set of effects that they can be I suppose.
It's so frustrating doing a really long hard quest and getting at the end chest.... oh.. +4 throwing axes with flaming on them. (and only getting 20).
Oh look, I get it, I get the reason why they're there and still classified as loot but I think that always having an exact 20 is silly. I think there should be a range from 20 to 100 in a stack inside the chest.
Anyway it'd be nice.