How many 2x4 will you need to build a 20x30 garage?

Wall studs are to be 16" on center in most places.

20 feet x 12 inches (# of inches per foot) = 240 / 16 (spacing of wall studs) = 15

15 x 2 (both of the 20 foot walls) = 30

30 x 12 = 360 / 16 = 23 x 2 = 46

Add 4 extra for the corners so 4 + 46 + 30 = 80.

That's just the walls. The rest will depend on any doors or Windows you have because those will require adding extra jack & king studs to carry the load as well as how you decide to handle the roof.