I have a greenhouse your size, I would top or super crop, lst. Or a combination of training methods. Wider not taller. I'm paranoid about one getting too tall and out of control. Bushes with lots of buds us what I'm going for.
You doing pots or beds? My GH is 7ftw x 7ftT x 20ftL and I have 30 plants in 30 gal pots. I vegged clones for about 4 or 5 weeks and flipped them. They more than doubled in size and the GH is full lol
I have a greenhouse the same size for light dep. If full season plants I'd only do 1-2 plants, but that's just me. I start early and use 300-700 gallon smart pots