In my experience, the number of NPC vessels of a certain type is determined by the cost (or possibly block count or firepower) of the blueprint. I set a cheap, TIE-Fighter inspired ship to be enemy useable, and now they spawn very frequently and almost always in groups of 2 or 3. The trading guild seems to send out the cheapest of its blueprints to patrol the area around shops, but when either the pirates or traders spawn actual combat fleets, they can and will include some larger craft.