So it is possible to give yourself 126 potions in one stack which I assume is a bug.
Steps to do it:
1. Run these commands into the chat: (I chose the potion of weakness because I was playing around with them)
/give @p potion 64 16456
/give @p potion 63 16456
2. Place a dispenser and shift-click the full stack of potions into it.
3. Take the other stack and click on the stack in the dispenser
4. You should have 126 potions in one stack