The median revenue per developer per month is an indicator of profitability and probably a better metric to decide the right team size.

So if you look at figure 7b, the sweet spot seems to be a team size of 6 to 9 people (where the median revenue per developer per month is $550 to $820. In contrast, the median revenue per developer per month for 10 to 19 person teams is $320 - $620).

If you can hire 19 people, you should probably split them to run two projects of 6-9 people each. :-]