Until the AI throws Missile Immune ships at you.
MLRS turrets and fortresses both use missile ammo so that's two of the best defensive structures that can't fire on missile immune ships.
There's only a handful of them, but I've taken to unlocking both Laser turrets and Basic Turrets Mk II as I find the missile immune ships more of a threat recently.
Especially Eye Bots. They've been the biggest threat to me recently, missile immune and cloaking, grrrr......
As for the OP's question, I do 10x MLRS and 10x Basic turret per wormhole and each of my three starting structure clumps, the 5x basic turrets as needed to cover all my harvesters so a single Raptor that gets through doesn't kill of all my harvesters before my FRD ships can catch him. I don't bother with tractors this early, I often don't even both with tractors at all, I'll use grav turrets instead.
D.