Is it worth adding a 2nd roof vent to an unfinished detached garage?
You do not need roof vents for non-insulated open roof garages.

FYI: For attics, the typical ratio is 1:150, so for every 150 sf of space then install 1 sf of venting. You have 576 sf so if you were to need them, you would need a minimum of 5.8 sf total.