6-10 plants will give you a lot of tomatoes.

If you want to plant more kinds of tomatoes in the same space, you can plant more plants closer together and keep them small by pruning or even using smaller containers.

Florida Weave is a good option to keep plants off the ground with little expense.

Here are some pictures from NCTomatoMan's garden:

Some pictures - the latest on the driveway garden

I grow intensively. SFG is just a name, you do not have to buy all that stuff to grow intensively. Some SFG gardeners even grow their plants in compost, but better I think is to read here, and in the soil forum, to get an idea of what plants need, and do what you can to provide the best possible environment for them.
I echo what Art said, that 6-10 plants will give you a lot of tomatoes even if they are determinate plants.
However, I differ in his approach in that I do not think pruning is advisable(especially in determinate tomatoes) if your goal is production and you have 400 square feet- more pruning means less tomatoes, and it does not make sense to grow more plants and cut production by pruning.
I've seen a beautiful picture right here of a SF gardener who pruned his plant and it had plenty of tomatoes, but it may have been indeterminate, with the space you have, you could get more per plant by not doing that.

This post was edited by lucille on Sat, Mar 29, 14 at 16:14

I agree Lucille about pruning. I was just thinking about doing that to try more varieties. Just a few tomato plants is probably enough for most of us, but suppose you wanted to try 15-20 different tomato varieties in a small garden. You could grow small plants and do it that way.

I grow my tomatoes (DET & INDET) in less than 2ft spacing, maybe 18 - 20 inches.

Spacing , mostly is to accommodate the top rather than the roots. Lets do a math: A 1.5ft by 1.5ft area with 1ft depth will have 2.25 cubic ft of soil or about 15 gallons(3 bucketful) . That is MORE than what the root system needs to spread, as long as the nutrients are in there. The extra spacing is really more for convenience of the gardener rather than the need of the plants.

ANOTHER IDEA:

I have read this not foo long ago, right in this forum:

PLANT TWO PER HILL INSTEAD OF JUST ONE. that is how cucumbers squash and potatoes are usually planted.

Here how to do do it:
In the same hill plant the two plants 6 to 8" apart.
Space the next hill (center -to- center) 30".

So in effect it is alike planting at 15" spacing but this method offers some convenience. I am going to try this method myself.

Let me say that there is nothing wrong with extra spacing if you have the resources. If nothing else, if offers convenience.