I run a 4 ton air conditioning i think i will be ok ...If you have the means to control heat I'd go with 8. But they get hot and throw a mean hotspot.
Fwiw most growers I know are moving away from Gavitas. Those greenhouse reflectors they come in suck for indoor gardening. Most are going back to either 600/1000w hps in batwings or vertical parabolics.
5 gavitas, this rooms about the same size. Invest in the wider hood ditch the one that comes with the fixture. The pic is with the stock hood. Your gona blow that shit up bro!!