A walled in surface crop plot will produce more than enough food to keep your little drunken sociopaths so fat that they can hardly walk, practically indefinitely. (You may need to add a second one, when you exceed 200 of them-- This of course, is in addition to the underground farm plot, and at a size of 10x10.)
By walled in, I mean literally walled in-- with a roof. Perfectly safe from sieges and flying animals that way. Will not impact the ability to grow food in it. You can more effectively get 100% utilization by smoothing the plot site, then flooding mud, before building the plot. Works like a charm.
Currently, Surface crops dont have any seasonal restrictions, so you can grow any crop on any season. That means you can grow strawberries in the spring, prickle berries in the summer, fisher berries in the fall, and lowland grass (or ropereed, or whip vine, or the coveted sunberries, if you can source them) in the winter, and have at least 3 crops that are directly edible, AND brewable on demand, throughout the year. Balance that with plump helmet in the winter, and there will ALWAYS be a directly edible plant crop in your fortress that is in season, and your little bearded bastards wont have any bad thoughts about always eating or drinking the same old stuff.
Just make sure that cooking seeds is turned off, and that you have properly configured your stockpiles (both for harvested crops and for seed storage in regard to efficient planting time), and you will be literally ROLLING in raw plant based food items. Set it up, and forget about it. Literally. 5 planters that get good at their jobs can feed the whole fortress effortlessly.
I abuse the crap out of the "no seasonal restrictions" thing on surface crops, since I can bulk process roprereeds (which have the same value as pig tails, which do have a seasonal restriction) 24/7, nonstop, through a clothing industry for trade fodder purposes. (also ensures that nobody gets nakedness related bad thoughts, as a bonus.) Couple that with ceramics and green glass, and you will never want for trade goods again.
Just one 10x10 surface crop plot, and one 10x10 underground farm plot, used at maximum efficiency, is all you will ever really need, barring some special industrial purpose. (Like with my mass-production of ropereed for clothing industry.)