My attached garage is 21' X 21' inside dimension. It had 1 bare bulb originally. I installed (4) 4' t-12 (1.5in DIA) 2 bulb fixtures. They were spaced 5' inside from the wall. I suppose if you have a 3 car garage I would go for 6 fixtures. The fixtures I got from lowes for something like $25. I will say that if your garage gets close to freezing the lights won't start. In that case the t-8 (1in DIA) should be considered. The reason I got the t-12 fixtures were that they are cheaper. I ran 12/2 (20 amp) wiring to each one. I think you can use 14/2 (15 amp) for strictly lighting though. So everything wiring, bulbs, fixtures, elec boxes and accessories came out to like $150. You might browse craigslist for cheaper fixtures and stuff.