Why would be impossible put a limit on the maximal numbers of units (and on maximal animals spreaded) based on population, number of cities, period (medioeval, ancient etc), golden age or number ofgreat general or civics etc? Perhaps with a great, great increase in revolution if you are over the limit? Perhaps setting a higher limit if you are at war?
So you could stay below the maximal units number consented by game, and could be useful to reduce time between turn and max memory usage.
Sorry if it is a stupid question.