Each cart you send counts as a march.
So the real way to think about this is you've reached the maximum number of marches.
Upgrade the Council Hall to increase the number of marches you can have. You get an additional march for every 5 levels you raise it to.
[3 extra marches via Studies: Training / Tactics I (2 levels) and Assailants / Tactics II, as well.]
Don't forget, attacking Assailants and Inquisitors count as marches, as do sending carts / resources to others, and attacking tiles or towns.