Why *numbers* of slots? I would speak of slot *types*. The railgun is an idea I've been playing with for a while in my mind. Most medium to large ships would have a railgun built into the hull, running along the center of the ship's body. It would be able to take out a capital ship of a single shot, but, to become operational as a stand-off weapon that it is, you'd have to get a number of upgrades: At least 4 synthetic aperture passive radars, a synthetic aperture extender interface that allows 4 such radars to operate as a single unit for ultra-remote sensing, a telescopic view-finder, a very large capacitor, and a gyroscope, for very fine control of ship attitude with all thrusters and jets turned off. When you fire a slug, the capacitor is completely discharged and the power draw for the next minute or so is so high that lights in the ship dim, the fans slow down and the computers and HUD reboot.
Anyways, some ships would not even have a liquid helium cooling system; others would. Some weapon slots would have a coolant connection, and these would be compatible with weapons that require it.
To be autotracking compatible, a weapon slot would have to have a wider flat surface with threaded holes for bolts, extra wiring, a fiber optic channel so that it can receive fresh target information, plus a sensor slot right next to it so that it can sense inaccuracies due to torsion of the wing, and make corrections to the info from the main targeting computer, on the spot.
Thus, cheaper ships would simply not have the facilities necessary for autotracking expansion installation. And of those ships that do, some might support larger autotracking cones than others, and some would support larger weapons than others. At the lowest level you could get a ship with 2 autotracking compatible mounts, but they might only support light weapons and only a 2-degree autotracking correction cone. Largest cone should be 4 degrees, to be realistic. At the highest end, you could get a ship with 4 autotracking fixe wing mount slots supporting large size guns, and with coolant facilities to boot. Not that I wouldn't rather have turrets, myself.
Anyhow, I'd much prefer to qualify things like mounts by detailed attributes than have some magic numbers associated with them. Configuring one's ship is probably the most fun part of the game, and I'd like it to be a science and an art, no simplifications at all.