It doesn't have to scale well, the enemy's action economy doesn't scale either. It's a boss rush.
I want number of units you deploy to be mostly a matter of taste. More units are useful because of their reaction attacks, but they get in the way of each others' movements, trigger the protagonista's "Necessary Distance" flaw, and it's hard to protect all of them at once. I figure 4-7 will be best for most bosses.