Depends how much you want to paint, I guess! It's pretty cheap though, so a 4oz bottle wouldn't be a bad idea and it'll last for a long time.
Just remember that less is more! You can always go over with another coat if some of the leather is showing through. Once you get too much that's when the paint can start to crack.
I never purchased one but the best way to paint overall color on a boot was to get an air brush, so it was a really light spray. And then you just tape the sole and the heel/edges of the boot that you don't want painted. I usually just tried to find a spray paint that matched close enough to the shade I needed, although I did custom color match the turquoise boots with the dress. I painted them whole darned things with a paint brush! But they matched perfectly.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.