I fi had to guess, I would suggest that maybe 1 person out of 500 actually has a round first knuckle. The rest of us are "normal".
Out of that 1/2%, I'd also suggest that less than 1% of those still have a round thumb while pressing with the thumb to hold onto the ball.
(If someone says you should use no pressure to hold onto the ball, I will call him/her with either a liar or a fool. There is no such thing as "zero" pressure to hold onto the ball. There is adequate and minimal pressure/force/muscle and there's too much pressure/force/muscle. )
Once you press your thumb against the inside of the thumb hole, I'd suggest 999/1000 people have an oval thumb and should be using an oval thumb hole. The degree of oval-ness is relative to the size, flesh, bone, tendon, ligaments, and muscle of the thumb and the hand/forearm doing the holding.