Since the basic control of the game is not obvious, so I hope this guide can be of help.
Player will have 4 Potion slots to use, each can hold 5 Bottle for the total of 20 bottles.
You can swap potion slot by pressing  on your keyboard.
This also apply to weapons as well, you can equip and swap between 2 weapons by pressing 
Potion have two grade in total, the small one cost 7 souls for each and the big one cost you a whooping 30 souls. So use it wisely.