How many 2x4's needed to build shed 18x20?

If you are talking studs only and spaced at normal 16" o.c. at least 48 and including 1 or 2 extra to make your outside corners properly figure about 56 to 60. this does not include double top plates and single bottom plate which should be as long as practical. these will total about 192 lineal (actual) feet.