I have a dual 400w, circular light mover. it adjusts to any radius you like, from 2ft to 8 foot, but potentially more...rater just have two circulars then, lol.
you can see videos here
http://www.youtube.com/onenumcat or read w/photos here
https://www.rollitup.org/grow-room-design-setup/155955-wind-powered-rotating-dual-hid.html
I'm growing 25 plants, LST's to save headroom...no problem! I guess you may only get 18 depending on strain, growing methods, and your own personal growing style.
I have 25 in a 6wX10dX8h room, but I'm using a 12x12sq table to grow on atm. when they're bigger, I'll remove the table, use the floor and spread em out.
also, using mh
only, for the main veg, swap a mh for a hps just before flowering starts(about 2 wks), and, finally, about mid way through the flowering stage, replace the remaining mh for another hps.
but I've got 6 strains, flowering from march 1 to june 1, so I'll have lots of time with dual HPS for some dense nugz!
just a bonus to using a circular light mover with multiple bulbs...not only do you move the light, you mix it also