Each character can have one buff active per exclusive category at the same time. The exclusive categories occupy independent buff slots, so you can simultaneously have:

• 1 Potion — only one potion in effect at a time
• 1 Elixir — only one elixir in effect at a time
• 1 Stronghold Food — only one stronghold food may be in effect at a time
• 1 Event Food — only one Event Food buff at a time

Scrolls, Belt Items, and Other buffs stack independently — you can have many of those active at once.