Err...

I'm not sure of the questions and anxswers, there, but I think there are two notions:
1/ In any TW games the number of units simultaneously on the field is limited. As far as I kniow, 16 for MTW:VI, and 20 for RTW

2/ In RTW, I have noticed that sometimes you have two small armies attacking another army: in this case, one of the two armies ios "reinforcements", and comes to the field sometimes 10 minutes after the start of the battle, even if the total number of units of the two armies would've been 10

Is your reinforcement problem cocernig two big stacks, or sometimes two small stacks?

@sapi: is this stteing in M2TW a setting, letting as many units as you want on the battlefield?