Minimum of 4, max of 7. 5 would probably produce a bit more in total than 4 but at that point 6 or 7 would probably produce about the same as 5 because you'll be running out of light catching area, so I'd go with 5 unless you're worried some may start out slow or be runts, in which case toss in another 1 or 2 so if they don't take off the ones next to them will just fill in the space instead.