Because it is made even with the forelock
Because and even amount makes the neck look wonky. I have braided a horse in 12 and it looked bad but when I did 13 it looked great.
As with so many things in the equestrian world - mostly because of traditions that people hang onto.
You don't have have to more of a tradition and the forelock makes it even it's like how they say you should plait main on the right hand side something to do with the olden times I think and they have just stayed
The forelock counts as the last one making it even
Apparently it is because it gives the appearance of a more balanced neck and prevents people trying to divide the neck into 2 halves :)
Regardless of the number of button braids, there are no specific counters that are recommended !! Nevertheless your button braids must be done regularly and it also depends on the mane of your horse if it has a thick mane or not😉
The people say you have to make an odd number (ideally 13) and if you make a button braids on the forelock, you have to make a tail braiding !! :)