I wouldn't be able to run more than the eight with my power as is. Plus this is at home and I'll hope to venture out to cheaper power and bigger space if/as needed. So 3k would be plenty and the 1.6k cfm would be about the minimum then?
are they all s9's? I wouldn't want my farm to have "The minimum amount of cfm exhaust" I would rather have more than enough to accommodate for the few days of the year where it might get extremely hot outside. Calculating the CFM on these really isn't like hard numbers, by that I mean the fans on the machines are variable speeds. At full speed the s9's move almost 300cfm, If you take that into consideration 8*300=2400CFM. Thats worst case scenario (I would build it to those numbers), the colder the intake temp on the miners the slower the fans spin meaning the less cfm they actually move.