In our 20' goosenecks, we haul about 10 head of 1000lb cows, without too much crowding. Borrowed the neighbors 24footer and had 14 in it, but it was 8 foot wide instead of 7.
They need good footing, so if it has smooth rubber floors mats, consider using sawdust, as once they relieve themselves, it gets very slick (also makes for a nice car wash for all the fords you pass).