I should have donkeys? How many?
As many as troops and as many as goods or crates (not sure which). Will be quite expensive though
So for example, 100 troops, 500 goods, 500 donkeys would be full speed. 500 troops, 500 goods, 500 donkeys would be about the same speed, slightly lower. Then there are lots of variables to this.
500 troops can carry 500 goods of any type. So for example if you buy goods in five villages, you can carry 5*500=2500 goods on 500 donkeys without slowing down at all.