I was just playing Advance Wars Dark Conflict and hit the unit limit of 50. I have played all the previous titles, and do not recall ever hitting this limit while playing them, is Dark Conflict the only one to have this?
All Advanced Wars game I've played, Dual Strike and Black Hole Rising, have this limit. I'll assume the original has it too. It's usually very difficult to hit this limit while playing campaign, war room and versus against an AI, but you can see this limit when designing maps:
The left is the number of units you can place (50 since there are no units on the map currently), and on the right the number of properties left.