A quick question: can the Visions card only allow one spell to be discarded per upkeep? Or can several spells be discarded?

Looks like you're referring to the old edition of Visions.

It should just be that one investigator discards one spell per upkeep. Otherwise you could have someone turn their entire collection into a cache of clues.

Think of this as similar to the Science Building's dissect ability: to crunch two gates, you have to use it for two turns.