you get more than one (though there are some where it's not possible to get more than one) and combine them while they're in your inventory. Like if you had two fireworks magic and combined them (you can't be wearing them when you combine them, they have to be in your inventory) then you'd have two star fireworks magic.
My question is, what does it do when you combine them? Does it make them last longer? Recover faster? I've never had a magic with multiple stars...