Why? You have a really, really, big spaceship. And then only 30 turrets? That is very unrealistic. Especially since larger turrets use more slots and in the end you don't have 30 but only 15 or so.
The correction nozzles show the way. The bigger the spaceship, the more correction nozzles you need. However, there comes the point where you lose agility. Since it is no longer possible to place more correction nozzles. Or you can't put anything else in.
You can do the same with the turrets. Volume is only one possible indicator. The number of slots is not scaled linearly, but in a curve function. The more volume you have, the more volume you need for another slot. It then comes automatically the point where more volume no longer effectively delivers more slots.
There's one thing you mustn't forget. The bigger a ship gets, the slower it gets. This makes it easier to hit and also provides more area to hit.
From the modules' point of view, only one thing can be said. Such a system can even accommodate 2 types of modules. Namely comparable to the modules for the storage room. One module for a fixed number of slots and one that provides more slots in percentage.
Alternatively, a completely different system. Got one in my mind. But would completely change the ship designe and requires a new block with a new mechanics :)