I would prefer a larger map than population. Game borders are the crux for me not number of mobiles on the map.
Eventually you could include population scaling. One character represents more than one head of population as a ratio, with the population ratio discretely expressed in the GUI. Players should also be able to influence this, setting higher ratios if they need to preserve fps.
Ratios are not strict, so a settlement which has achieved 1:15 ratio from 1:14 will have less figures walking out of huts than walk in. yet some characters might have a fixed ratio, soldiers for example might be fixed at 1:10 once population ratio exceeds that, and your Ensi or Tribal Elders might always be single characters.
In any event player should have control over the ratio in game settings, with recommendations dependent on CPU and GPU detection.