I think there should be differed between turret and point defense turrets, as I don't see a manned point defend turret against missiles and possibly fighter a good thing, as ships need way to big crews, if it is a rule like one gun per man. And if you don't want big ships to be piloted by one thing, as I don't want to, than there is plenty of roles to fill in. Not all on military ships are for using their guns, there is the engine, fire control teams and such. And even automated weapon system should have personal watching it function correct. So even if we want it to be sci-fi, we still have reasons for being more people on big ships and I don't even talk about having carrier capacity and dozens of fighter wanted to be manned.
Also if ship battle isn't stationary, like being side to side and firing the crap out of each other, then at least one person should only be focusing on maneuvering that big metal junk, so it isn't rammed into an asteroid, or other ships.
As an example, what I would think about gun crew is, that let's say one person could use five guns, but they all can only target one ship. This way, we have "gun crews" in some way, for it to use whatever button it is to automatically fire the turrets, but we also don't need hundreds of people to operate each ship. So that even if two organizations are fighting each other, that people who don't like pvp/firing guns can still do other things, while the other can have their fun in destroying each other’s creation. Meaning, we can have even in war times have a variety of play stiles and have more fun.
Completely different is it with small fighter like vessels, there should be fixed guns for those, to only shoot in one direction, then they could be only with one person, but if they have a turret, then there should be two person in it.
This all is, if the battle system is somewhat dynamic, so that the ships are moving and not just stand stills in a fight. I would prefer it that way, as it sounds more fun to me this way, and would add more immersive to the game. Also I think, with how firing weapons work in dual universe it could get pretty boring, if 200 people on a huge ship are just hitting some buttons for firing if they have to repeat the progress for a long time.