If you are able to work on all sides of the bed you've made, I would put 8 indeterminate (tall) tomato plants in there, placing them in 2 rows of 4 with the 8 ft stakes that they'll need pounded in at the edges of the bed and the tomatoes placed right against them inside the bed, if that makes sense. In other words, say you built this bed to a height of 1 ft with cedar wood, the stakes would go right up against that wooden frame so they're still INSIDE the bed, but just barely. As the tomatoes grow, they'll produce sucker branches that you can pinch if you need more space or let grow to get more tomatoes. You'll have a nice pathway in the middle of that bed to make harvesting within the bed much easier. I would use soaker hoses and cover with about 6" of straw to keep that pathway nice and dry and also prevent water from splashing onto the lower leaves causing disease. Each week during the growing season, I would tie those tomatoes to their stakes with soft cotton twine.
If you decide on the shorter determinate or dwarf plants, you could put many more plants in there and treat them as you would your pepper plants. Determinates grow to 3-4 ft and Dwarfs are even shorter, 2 ft tops.
