The short answer
Treat them as a gate, not a target. If your magic number is comfortably above one and CAC payback is under twelve months, the machine is returning more than you feed it and hiring is the obvious use of cash. Between about 0.5 and 0.75, adding reps mostly buys you a bigger, slower version of the same problem, so fix productivity first. Below 0.5, hiring makes things worse. Two adjustments people forget: measure spend in the period before the revenue it produced, since a rep hired today produces nothing for two quarters, and use fully ramped productivity rather than team average, otherwise every new hire silently drags the ratio down and you conclude the market is saturated when you have simply hired too fast.