Weight load of a hanging 2x4 shelf
There's no way to answer this (what will the shelf hold, weight-wise) without knowing what your garage ceiling joists are, if they are part of a truss or just joists, how long the span is, and really, what they were designed to hold up in the first place - many garage ceiling joists are barely in spec to hold drywall, much less a load...

A FAR better attachment method is to place a beam (or more than one) inside the "attic" space across the top of the joists, spanning several, and run threaded rod (steel) through that beam and down through the ceiling to support the item below. In this case it would mean you can spread the load across many more joists (by using a long beam in the attic space.) It also means that you are not depending on wood screws to hold the load directly.