I think it depends on where you are on your stacker journey. I’m fairly new and started at the default amount (can’t remember what that was). I suddenly found I didn’t have many sats as I was fascinated by so many topics. I reduced my zaps to 10 but I feel that as a new kid on the block that 21 Sats to zap is a solid number which is my current default amount. I’m giving a nod to history along with being a decent amount.