The Lords of Madness book states the dietary needs of an illithid.
A mind flayer must have a minimum of one fresh brain per month. Any less than that and it suffers physical debilitation, becoming so weak that it could die. Its ideal diet is one brain per week. A mind flayer that consumes one brain a week does not feel deprived. It can eat more than that for enjoyment and for the psychic boost, and it will if brains are plentiful.
Given those numbers, what ratio of humans to illithid would be necessary to prevent starvation, and what ratio would be ideal for illithid? Assuming that they’re not getting new captives, just breeding existing captives to replace the ones they eat.
I would like the answer to be detailed enough that I could calculate ratios for other races that are capable of reproducing faster or slower than humans, as well.