Book of about 40 in northern hemisphere, another same same in southern hemisphere at the other end of the year. All thoroughbred foals have to be ‘naturally’ sired - no artificial insemination permitted. 40 leaves a bit of wiggle room for a return service if the mares involved didn’t catch on the first try. Also, there’s a huge advantage to a foal bred/born early in the season cos they’re older and stronger when they tick over into 2 year old status and can start to race. So there’s a practical limit to the number of mares that can be bred by a stallion in a given year. This is factored into the price of a stud fee for a top rated stallion. A mare can produce maybe 10-12 foals in a lifetime, a stallion is much more productive, and profitable.