Assuming 2x4 walls with a half inch layer of siding at 8 ft high, to make math easy, you have 4" thick walls. If you're perfect at tetris and stacking your materials, you have about 6.7 cy of wall not accounting for any openings. Assuming a 2x6 rafter with half inch decking, not accouting for joists or shingles, a 12/12 roof pitch and a gable roof gives you another 7.3 cy of structure. So perfectly stacked you have about 14 cy of structure, not considering the space of the garage door, joists, or materials not stacking correctly. My late night math, (which could be WAY off) I'd say depending on price and time, two loads would be more than enough if you don't just toss your waste into the dumpster.