read label on the shingles. cheaper ones are three bundles per roofing square, more expensive ones [heavier duty -- last longer] are four per roofing square. there are 100 sq ft in a roofing square [10x10]. Roofing needs MUST be measured on the roof, not on the floor plan.
so you're right at 500 sq ft = 5 squares. plus 15 percent for waste, starters, etc. = 5.75 squares, so either 18 ligher quality bundles or 23 heavier ones.
you'll also need 30 pound felt [tar paper] of other waterproof layer under the shingles.
grampa -- i've done my own house a couple of times