About 15. And sure, most I barely use. But sometimes I want to use them and they're in my bag and not in my wallet. This would be a great solution to that problem for me if it could hold all of them.
Even though Coin doesn't solve this problem, especially since it doesn't have EMV which is required in Europe, I'd love to have all off them one card. I have another boatload of cards at home that are rarely / never used because I never have them on me when I could use them.
So basically: very cool idea, needs some more thought for the european market though.