1 still easily accomodates 20 to 30 dwarves, give or take, depending on the skill of the brewer.
If your brewers suck, you may need 4 or so. (This is strongly dependent on how fast you need to make booze. Say, the RNG drops migrants of doom on you, and you go from 7 to 60 dwarves "instantly". You are going to need booze, and fast. 4 stills going full tilt will resolve the alcohol emergency quickly, but 2 to 3 of them should be removed soon after.)
Dwarves do not eat seeds unless they are cooked. Never cook seeds. It's wasteful.
To brew, you need:
1 item to brew
1 storage container (pot or barrel)
1 still
1 brewer.
Most booze is made from plant items, like plump helmet, strawberry, prickleberry, ropereed, etc.
Brewing takes 1 plant, produces a brewer level dependent qantity of booze in a barrel/pot, and 5 seeds from the plant used.
(Other kinds of booze can be made from other materials, like honey. As a novice, I suggest staying away from the very buggy bee keeping system. It's full of headache, even for experienced players.)
Make sure the plants are enabled for brewing in the kitchens submenu of the (z) stocks menu. Brewers will ONLY brew plants that are enabled for brewing. Same thing for cooking. The first thing I do before starting cooking is set restrictions to better manage food production.
Dwarves have personal preferences for booze, but this just affects the happiness boost they get from the drink. They don't specifically need that kind of drink. Dwarven wine (plump helmet ingredient) will solve their alcoholism needs fine, but the do like a little variety.