When need some extra cash, I also groom for the riders where I board.
Back when I was taught to braid, I was taught its an even number including the forelock.
I don't sew my braids in but I find that they stay in quite well. I find the braids turn out neater if you start off by banding them first. And always make sure the part is straight in between braids, you don't want it to look mish-mashed and bodgy.
Most of the horses I braid have about 10-16 braids including the forelock. I think the best tip I can tell you is to braid to make the neck look good.
I've never braided for a hunter, but from the researching (googling) I just did, the more the better.