So I read somewhere that you need 1 horse/donkey per crate and you'll keep the 44ish mpm.
Since all the trade goods seem to weigh different I usually end up with 200-300 crates for 350 ponies on my first buy.
(say 300 goods put me at 300 crates) If I bought 50 more goods, using the trade goods to crates ratio I received, the crates skyrocket past 350. I don't understand! Another issue is sometimes I'll be closer to my max crates, but not over, and my speed will drop to 24mpm.