Game Won't let me Replace Cards [Mulligan Issues]

[DocOdy]

I am not understanding for seeing a pattern for when I am the one chosen the replace cards and how many?
Is there somewhere that explains card replacement rules in detail?
Im guessing it is multifactorial including coinflip and leader???? but not sure

 

[Vaishar]

Leaders have a number on the card. That's the number of cards you can replace through the entire game. It's not x cards/turn anymore.

And the player who goes first gets another replacement besides the number on the leader card.