I run RDWC and 1 plant from seed fills up a .6x.6 meter area SCROG around 5 weeks from pop. That varies a bit depending on pheno and environment of course, but that's what I go by. You can veg for longer to fill up the screen but I wouldn't flip at anything less than a month.
In a square area I've found that square numbers of plants work best, 2-4-8 etc., it makes it easier to avoid clustered or sparse areas in the screen.
To answer your question though, no, I don't think 6 will be too much for that space.
Are you planning on doing 2 rows of screen with center access or one big square? 2 rows of 3 plants with walking space in the middle is what I would do in that space.