Check the Dustin Browder interview. He says that there were too many conflicting choices in WoL, and that was due to there being ~20 units in the game. With 10-12, it would limit choices enough so there isn't just an "implied choice" where there's one that's stronger and one that's weaker. However, there would be even more choice with all the evolutions, in such a way that depending on your choices, each swarm would feel and play different.
This makes it a lot more feasible for them to have ~20 missions, without the majority being "introduce X unit" levels.