Originally Posted by RAI

Ah no wonder why, I'm approaching level 90 as was wondering why I was still capped at 30 Allies,
of course all my stamina is spent on events and the WHY WON'T YOU DROP dragon quests.

Who needs story

You'll gain an ally slot for every 10 levels you gain after level 100. Once you hit level 150 you'll have 35 ally slots. Then, once you hit level 151 you'll have 50 ally slots. After that you can expand you ally slots by completing story quests.