Do I need one studio monitor or a pair?

A pair, unless you mix mono content exclusively. Stereo placement, panning, and reverb decisions require two speakers in an equilateral triangle. Prices you see on monitor listings are frequently per-side — a “$180 monitor” is a $360 pair, and we price every chain accordingly. If budget forces one box today, buy the first of a pair you can complete, not a single larger speaker.