Key to Kingdom rewards RE hearts chain of memories?

How do I get the key to the Kingdom hearts chain of memories rewards for PS4? When I first set foot in Atlantica, I got it. Otherwise I never got it, how do you get it? And can I go back to the other worlds / floors to get the keys to the bonus there? And how many can you get per floor / world / room? And how many can you keep in inventory at the same time?

You can get them from the 6th or 7th floor after a long fight I think

I'm on the 7th floor. Got it right away when I got into the first fight xD, do you happen to know how many you get per floor and how many you can keep in your inventory at the same time?

You can only keep one in your inventory, but you can use the card in any world and I don't know how many you can get

But can you get more than 1?

Yes