This depends upon the base fertility of the region and the distance to your (future) capital. You can move your capital, but I found it easier to do so early and then never again and to plan accordingly.
Personally, I do not build any farms if the sum of base farm level, existing farm bonuses and distance penalty is equal to or greater than 6. If required, I raise the population by disbanding cheap units (peasants) and mercenaries in the respective city.
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