The number next to the units on the sheets is the initiative. Each shet has 5 "slots" (although in the base game, one of these is always empty). The top slot is Initiative 1, the next is 2, and so on.
A synopsis of the base-game units:
Initiative 1 units: Daqan Bowmen, Latari Archers, Waiqar Skeleton Archers, Uthuk Flesh Rippers, Neutral Razorwings, Neutral Sorcerers
Initiative 2 units: Daqan Knights, Latari Pegasus Riders, Waiqar Necromancers, Uthuk Berserkers, Neutral Beastmen
Initiative 3 units: Daqan Footmen, Latari Sorceresses, Uthuk Warlocks, Neutral Hellhounds (Waiqar don't have an Initiative 3 unit in the base game)
Initiative 4 units: Latari Warriors, Waiqar Reanimates, Neutral Dragons (Daqan and Uthuk don't have initiative 4 units in the base game)
Initiative 5 units: Daqan Seige Towers, Waiqar Dark Knights, Uthuk Chaos Lords, Neutral Giants (Latari don't have an Initiative 5 unit in the base game).