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.
Cart limits are typically determined based on usage patterns, account activity, or system safeguards to prevent misuse; however, if you're consistently reaching your limit and need more access, you can often request an increase by contacting support or your account manager. For example, companies like Corporate Transportation Solutions may require multiple carts for scheduling client pickups or organizing event logistics, and they can apply for expanded limits to meet business demands. If you're receiving a message stating that the maximum number of carts has been sent—even when you haven't used any for days—it may be due to shared account usage, automated system errors, or previously queued requests; reaching out to customer support is the best way to resolve this and ensure your access is restored.