Generally, each dwarf eats twice per season, and drinks four times per season.
A legendary farmer almost always produces ~4 plump helmets per farm tile (as in, 1 1x1 farm plot) per harvest.
You can typically have about 2 1/2 to 2 3/4 plump helmet harvests per season. This works out to ~10-12 plump helmets per farm tile per season.
So, you could support one dwarf for every tile of farm plot that you have, with some excess. A single maximum-sized 10x10 farm plot could feed and booze about 110 dwarves. The only problem is that this can take some careful management to make sure that your brewer doesn't brew too many mushrooms into booze, causing your people to starve.
Of course, you could also arrange for ALL of your plump helmets to be brewed, and then some of your booze cooked into meals. This would essentially make each dwarf take up 6 dwarven wine per season (2 eaten as meals, 4 drank). Your production with a legendary farmer, then, would be ~50-60 combined food and drink per farm plot tile per season, essentially meaning that every tile could support ~8-10 dwarves. A 10x10 farm plot would then feed 800-1000 dwarves. This, again, takes more micromanagement, however, to make sure your cook doesn't cook all of your booze.
Generally, though, I find that I have more than enough food from 49 farm tiles (as in, 1 7x7 farm plot, though usually divided into several smaller ones) that are left fallow most of the year. Food production is very simply in DF because farming is almost unreasonably productive.