You can increase your March Capacity via upgrade of Castle, research projects in Combat, talent point assignment in War category, and the Airship ability March of Fury. There are always some items with which you can temporarily increase it.
There is a March Slot III project available further in the Combat Tree which can be researched once your academy reaches level 20. Another March Slot becomes available when your VIP reaches 8 (and you have your VIP active).