That's 6 x 12? Must be one of those optical delusions......doesn't look that large in the pics.
Tomatoes take a lot of space. You'll likely need to add some trellis or other supports....then if you are successful you'll need a ladder to get to the top of the plant in such a tall bed
From:
http://kgi.org/?gclid=CjwKEAjwy6O7B...965m1uCYA8DxwCAzZ38SUifQQdgcqu3myuxoCu-nw_wcB
Ideal
spacing for home garden
tomatoes is generally 24 to 36 inches between
plants.
Planting closer than 24 inches reduces air circulation around the
plants and can trigger disease outbreaks. Large-vine
tomatoes should be spaced 36 inches apart. Rows should be 4 to 5 feet apart.
Taking this info into consideration...I'd say 3 per bed....4 at most.
Are you planting determinate or indeterminate plants? (hopefully heirloom varieties)
We have about 25 in the ground here. Amos Coli...Berkeley Tie Dye...Cherokee Purple, Green Copia, Black Krim....
good luck