Depends on other factors or how many you kill in the process.
With my sativas, I only go four plants in 15 gal pots at most. And That's somewhat crowded
But with indicas like my blueberry it's short doesn't get too big so I'd do six prob undwr a light
Same with my lav kush, it gets tall but stays small and compact so I could fit six under a light in 15 gal pots.
I veg em to be 1-2' tall (sativas 1') (indicas 2') and flip em. Never top, only cut off the suckers for training
So it depends on veg time (how tall u allow the plant to be before u flower it) and also genetics (sativa/indica)
Sativas when flowered will stretch wayy more than indicas. My agent orange flowered at 16" finishes out 50-60" tall
My blueberry flowered at 16" finishes out at prob 22" lol
My lav kush flowered at 16" finishes out at 48-50"
I don't think rockwool cube size is the variable to be concerned with. More personal preference. The variable to be concerned with imo is genetics and height of the plant when flowered
Hope this helps