Hi Vandervecken!
Backpack items don't stack I am afraid, so each potion that you have (even if they are of the same type) takes up one space. So, if you had three Pots of healing, they would take up 3 spaces.
Note that some potions have multiple uses however. Even though a potion takes up one space, it may have more than one use. For example:
Troll's blood (2 uses)
(backpack)
Restores 2 health at the
start of every combat round
for one combat
Note that herbs and roots do not take up backpack space, so you can have as many as you want (simply make a note on these on your hero sheet).