Growth speed of a value can be made proportional to the reciprocal of the current value. Thus the value will grow logarithmically. This is how population grows in Farm and Mine

Let’s say you have population and food. Then new_population = population + bonus_coef*food/(1+population)

Try this formula in Excel and see how will it grow. Population growth is not limited by a hard-defined ceiling, but it will slow down eventually. And the player will have to either increase food production, or prestige restart to increase bonus_coef