Practice and biomechanics!
Before I did P90X (trying to recover from an injury) I could barely manage 3 or 4 proper wide arm pull ups.
At the end of 3 months solid of doing them, depending on type obviously, I could do between 20-50 without too much effort.
I used to do one with a towel which was a semi-one-arm pull up. Couldn’t get close to half of one when I started, finished up being able to do 10 at a time.
P90X involved using your tip toe on a chair out in front once you were unable to do any more “proper” ones. I think I’d do on average about 100 in a session, so you tend to get better at them biomechanically more than anything else.