you can get away with 4 easy, depends on how often you want to fill your resivor honestly. i could grow two plants to flower with a 3 gallon resivor. right now i'm using it on 1 mother. could proboboyl actually flower 4 smaller plants with it, depends on how compact you want to go and if you can fill the resivor every day, heh.
i flower with a 18 gallon resivor, i had 6 plants on it at first, then i made a flood table to use with it and could probobly fit 9-12 max. right now i have two in it and with all the rootspace room they went from 8" tall when 12/12 was set to 3 feet 4 inches tall. that is more than triple in size, and it's not stretched badly.