the amount of food you can hold seems to be limitless... the amount you can produce isn't. A lvl 10 city with no valleys has a production capacity of 40,710,000, and an hourly production rate of approximately 480k per hour. That will vary according to research and hero's political level. This is all with no additional upgrades (replacing the one lumber/iron/and stone mills that comes standard with a 10) and with no valleys.