The 15% annual profit is just amazing. There is just one limitation: The max amount you can stake is 20 BTC (too much in my opinion. I wish I had that much!)
I don't think that is what Blackjack means there. They mean that the maximum amount accepted by all collective members who want to stake their coins is 20 BTC, so whenever they reach the cap of 20 BTC from all users staking their coins, then new players won't be able to join anymore to stake coins.