I have heard that willpower is finite and every person starts their day with a limited capacity.
On the other hand there are people like gym goers or sportspersons saying that they can muster up more will power if they have the right mentality at the right time.
I don’t know which one is right, but is it possible to increase one’s capacity for will power ? If so what kind of exercises or training methods do they have to use ?
Willpower is just an abstraction. It is not a “real” thing, consumed once you do something you wouldn’t.
That said yes, you can train it. Or rather, condition your behaviour so you do things that you’d otherwise avoid. The how-to is actually simple:
Eventually you won’t need the reward, but the behaviour is still there.
What you consider a reward is up to you. For example, for me snacking on cheese is a reward, but it might not be for you. With the right mindset, even mundane things can be a reward, like taking a comfy shower or playing some games.