Since YASS has only one game mode and one score in the game, it makes sense to have only one leaderboard on Google Play Game Services. Leaderboards are managed from their own tab inside the Game services area of the developer console.
Unlike achievements, it is not mandatory to have any leaderboard to be able to publish your game.
If your game has different levels of difficulty, you can have a leaderboard for each of them. This also applies if the game has several values that measure player progress, you can have a leaderboard for each of them.
Leaderboards can be created and managed in the