I usually have them about 3-4inches long ( a hands width), and i've done anything from 9 to 21 plaits depending on the thickness of the mane, and the length of the horses neck for racing and hunting TBs. There's no set number really, just make sure they're not golf ball sized or too small. What are you plaiting for?
I keep manes roughly from the tip of my little finger to the bottom of my hand - about 4 inches. I plait according the neck length, so more for a short neck, less for a long neck but always make sure I have an odd number on the neck, with the forelock making it even.
My chap has a very thick mane despite regular pulling and plaiting is difficult even with the mane short - don't suppose there is an easy answer to that.