if you are force carbonating every keg then i would say 6-8, I try spunding and purging all my brews and kegs so it is closer to final pressure. my last fill lasted for 12 kegs.
Depending on your location and how much driving you wish to do. You could jump over the river and check out Down the Hall Homebrew in Bellville.
https://downthehallhomebrew.com/