Good luck on your journey!! Imo, a true scrog with autos is kind of pointless. With a photo period grow you can top your plants several times and allow it to grow into, or fill out the net prior to flower. You can also prune them to get a nice even canopy.. Not the same with autos. They don't veg long enough to fill out a net. It will almost impossible to keep an even canopy with 6 or more auto plants.
The spacing for plants is normally 1-2 sqft per plant. How many should you run? As many as you can properly care for at once.
And a foot note about Sohum.. It has enough plant food for about 5-6 weeks.. If you're doing water only, from seed to harvest in the octopot, then I'd start thinking of how/what you will want to feed in a month or so. In the photo period world, most people transplant into the sohum soil, or some fresh sohum soil right before they flip to flower.