One plant for a pot that size is the most you need, if you want to raise it for fruit. A bigger pot may be needed before the end of summer,too.
You might be able to pull some of the small plants from the outside of the clump without disrupting too much, and transplant to other pots, or give away to friends? As Sandy said, though, cut off most of them with scissors and just keep the one strongest plant undisturbed in that pot.
Tomatoes will really need a full day of direct sun, so if you are able to find a place outdoors in the sun for your plant you will have much better luck raising it to give you fruit.
