How many reps do I need for muscle building?

It's not primarily about the number of reps, but about the effective time under tension (until no further movement is possible despite maximal effort) of the largest motor units, or rather their muscle fibres. These motor units should fatigue, so that your force output becomes minimal. The training load should therefore not be chosen too high, because otherwise the metabolic fatigue is too small. At the same time, the energy stress shouldn't be too high, because this may trigger interfering molecular processes that can inhibit the rise in muscle protein synthesis. The individually required time under tension needed to exhaust the largest motor units isn't precisely known. Roughly speaking, though, a combined time span of about 90 to 120 seconds can be given.

Source: COACH Magazin (Markus Gsell) – Interview with Marco Toigo