Firbolgs have the Firbolg Magic trait (VGtM, p. 107):
You can cast detect magic and disguise self with this trait, using Wisdom as your spell casting ability for them. Once you cast either spell, you can't cast it again with this trait until you finish a short or long rest.
Does Firbolg Magic have one use that's expended when either spell is cast? Or does it have two uses, where each spell can be cast once?