You can find this information in your Command Center's Info dialog. 

Tap 'Command Center' building > Tap 'Info' button. 

The number of marches you can send out simultaneously is decided by the level of your Command Center.  Upgrade your Command Center to increase the number of marches you can send out simultaneously.

The number of units you can send in one march also depends on the level of your  Command Center.  Upgrade your Command Center to increase the number of units you can send out from your city in a single army.