With mylar, though flat ultra white paint reflects as well w/no hot spots, your effective grow space is about 5x5 but can be pushed to 6x6 depending on strain and other factors. Within this depending on ceiling height you could work anything from 16 smaller SOG (Sea of Green) 3 ft spears to 4 in a SCROG (Screen of Green) setup. And everything inbetween.
Depends on how long you want to veg for and how big you want your plants to be. I like to grow a few big plants rather than a lot of small because there is less risk involved. You could probably fit 20-30 smaller plants or 6 bigger plants under 1000 watts
I would do 6 big plants, a vertical 1000 hanging in the center, no reflector . grow them up the walls. Tie them back to the pot. i would use 10 gallon pots of coco. Veg them till they are about 3 1/2 ft then flip, hopefully the finish around 5-6 ft and you would have should colas from top to bottom. Yeild would be outstanding .just my opinion tho