I dont think that reducing the number of supermutants would eliminate problem of artilery at all.
Problem is, that muties can shoot immediately at enemy target that is in few hexes distance. Even if you would disable automatic fire of muties, the mutant leader is often surrounded by mutants so you can eliminate him because there is not any of his pixel that you can aim..
If you would reduce the number, existing dual log custom of merc leaders would lead only to situation, that more dual logger would be involved in artillery and effect would be almost the same. In fact, even one mutant firing rocket into the grup could cause big disadvantage for its victims, because of knock downs, collateral damge, losing AP, frienly fire, distracting... .
But i am not against mercs. Even current number could stay. But n.1 priority is to set conditions, that mutant leader have to actively control his mutants, not fire and leave them. This could be done, if you set that mutants or in general, all mercs and slaves, would be inactive (not moving, not firing) for cca 15 seconds after their entrace to CITIES (not random encounters) and fixed locations.
This would cause, that you would have to bring your muties before start of the fight (and you would have to control them or at least organize their position) and if you would bring them into the city after start of fight, you would need to enter safe location, or your mercs would be eliminated.
So current practice, where you just click on second computer (trust me, nobody plays muties leader as primary char) would be eliminated.
But even if muties could not fire for few seconds, they could evade attention, if they are dropped into the group of enemy (you need to eliminate them, it cost AP, ammo, it breaks the formations). But this could eliminate another change, if muties would become more expensive, they would not risk their lost.
Other possible solutions.. only high level chars could use high number of mercs and they would have special mercs karma.. if they lost many mercs, they would not be able to buy them, .... . But this still not solve artillery problem and even one artillery abuse is very annoying, because i am always anticipating good, fair PvP combat, nothing less.