Are you using tomato cages, t-posts, letting them sprawl on the ground?
According to the "experts" minimum spacing is 2 - 3 ft but, if you put tomato plants on a grid 2 - 3 ft apart, I dare you to pick them.
I usually grow indeterminents, place them 2 - 3 ft. apart, in cages, in a single depth row and they are hard to pick.
Crowding also encourages all sorts of problems: mildew, fungus, bugs... If you have any amount of humidity, space them well.