Upgrade the "shrine of Oden" or through knowledge.

any ideas how to upgrade the # of marches allowed?

There are 2 different knowledge to upgrade your no. of march 1 is in invaders and 1 is in training. By doing both you will be able to increase no. of march by 3 units and rest is on shire of odin. From lvl 21 onward it gives 1 march extra with every upgrade. If you want my opinion then no. of march matters only if you want to farm heavily but if not want to farm heavily then those knowledge are of no use to you. Instead focus on how to increase troops in a single march.