Couple of Ag questions, about how many people can a farmer feed
A single legendary Planter can probably handle about 50-80 dwarves. But it'll depend on the efficiency of your fortress design (how far he has to walk from seed stockpile(s) to farm plot(s)), how lazy/dutiful he is, etc. And this is also assuming you have other dwarves processing the stuff he grows (someone to bag the quarry bush leaves once Toady makes those actually start giving seeds again, someone to cook the quarry bush leaves, someone to brew the brewable plants).
It will also depend on whether you include textiles (pig tails, rope reed, etc.) for clothing replacement as part of the picture.
and if I have a 3x3 plot how many dwarves can it feed at max capacity, per season.
Best case scenario, growing plump helmets and ignoring the lack of variety, also ignoring fertilizer, you get 2 crops per season, times 9 tiles, times about 4 plump helmets per stack (Legendary planter) = 72 plump helmets. A single dwarf needs 4 drinks and 2 meals per season, so that's 3 plump helmets (1 brewed, 2 eaten raw). So your 3x3 plot handles about 24 dwarves. Again, this is a best case scenario, and not realistic in a long-running fortress. You'll need some booze variety. You'll also want to use quarry bushes for food when those are fixed; those give you 5x the food per plant (each plant becomes 5 leaves = 5 meals after bagging and cooking), at the cost of not being available in every season, and taking longer than plump helmets to mature.