YAML Config confusion

Let's say the config looks like this:

Code:

player1:
       block1:
             setting: true
       block2:
             setting: true
       block3:
             setting: true
player2:
       block1:
             setting: true

How can I check what's the last blockX for a player, so I can add the next one? So, if player has 3 blocks (the last one is block3), the script knows that it must add block4 not block1 or block5. So how do I get that number?