Well you could always look at the amount of conversions each sales person can generate and the capacity your company can handle. If your company has enough capacity and there is still enough market left to hire an extra sales employee without cannibalising on the current sales force conversions.
While there are a lot of details and possible scenarios to this question, in general what my experience has guided me to doing in this regard is the following:
With the existing force, given your offerings, are you deriving the most revenue per territory that you can? This can be measured using TAM (total available market) and SAM (share of available market) numbers.
If it is not, divide and conquer (deeper) would possibly warrant another person(s).
Also, if there are uncovered territories that exist, is the product saleable in every region or geography?
Lastly, would a solid channel partner (indirect sales) be a better choice instead of a new hire?