the stack number of potions doesnt appear in 13w42a snapshot
it did work normal in the 13w41b snapshot

how to reproduce:
1: get snapshot 13w41b
2: type /give (your minecraft name) minecraft:potion 16 16453
3:it should give you a stack of 16 instant health potions. but the stack number doesnt appear
4: now do /gamemode 0 if you arent in survival already
5: trow the potions
6: see that its only visual. and that you can trow it 16 times

edit: seem to only happend when the potion has an potion effect