3 is enough. Point is the limit is there so you have to do hard decisions when you prepare for battle (same reason why you can’t switch loadouts mid match). It forces you to overcome sticky situations by outplaying the enemy with mercs(and equipment) that might not be suitable for that situation, instead of simply switching to the best pick.
Also you don’t neccesary need all those classes every merc can do everything. Every merc can revive/repair/plant/defuse even damage the EV (granted focusfiring that thing down with SMG-9s is propably not much fun). Its just that some mercs are better at doing it than others and you have to decide wether you want that advantage or not.
I Personaly run this setup most of the time. A pure damage merc (assault/recon) a engineer, and an ammosupport. The only thing I can’t cover with that, is healing/reviving and I have to relay on my teammates to provide for that (luckily quite a few love playing medic). Ofcourse it can happen that I end up in a team that has no medics at all. But that just means we have to play accordingly take care of ourself a bit more and in general play more towards our strength (after all no medics means we have more of something else).