Plant count is entirely up to how big or small you want to grow them. Ive set up 4x8 tents for others to run 4 or 8. You gotta keep in mind that you won't really be able to get into a tent that size and do much around the plants and it can difficult to deal with plants in the back row.
8-10 small plants with a short veg would work. Personally I'd run 4 bigger ones in a row so I could reach everything by just rotating pots or whatever.
2 550s will be a tad on the lighter side of how much you'd want, but if you run them closer they should still rock that space.