ive just started making plugs, and ive been fooling around with mission computer. i am trying to make a purchase limit from a light blaster from 3 to anything else. whenever i change the limit, everything else works, but i can still only buy 3.

By default the light blaster has a max of 8 so I expect you are limited by your ship's max guns. You can get a larger ship, sell some of your other guns, make a plug to allow your ship more guns, or change the light blaster so it doesn't count as a gun.

ok, how do you make a plug that increases the amount of guns on a ship? ive already made it weightless, so dont tell me to do that.

The oütf section of the Nova Bible tells you how to make an item take up (or not take up) a gun or turret slot.

The shďp section tells you how to make a ship have more (or fewer) gun or turret slots.

I will add a helpful note here, two of the same outfit that is supposed to increase a gun/turret slot will not work right. The first outfit will work as expected, but the second, third, etc. will not add any more slots, though if the outfits have other attributes they will work as expected. (i.e. if you made a gun that also adds a gun slot, the first gun will add a slot as well as work as a weapon. The second will add a second gun, but will not increase the number of gun slots any more)

I do believe, however, that different outfits that increase the number of slots do work together. (so one Sigma Mount Reinforcement, and one Made-Up Mount Reinforcement would each work as expected)