Steem Monsters Questions

I just got into this a couple hours ago and bought a few packs already. I've read up on as much as I can find about the game, but I still have a couple of questions that I haven't seen the answers to.

In no particular order:

1. I get that combining cards will level them up, but are we ever going to need more than one copy of a particular card? (In Hearthstone you can put 2 of most cards into a single deck, for example.)

2. Is there a limited number of these alpha cards? Is there a verifiable way to see if they're going to sell out at some point?

3. Do we know how rare the various rarities are? How often should we see a Legendary card, for example?

That's it for now! Here's a couple of screenshots of my good cards so far, just because they're so pretty. :)