You could actually keep 3-4 in a ten gallon. When they are younger they like only 4-5 inches of water, but as they get older, you will notice that they become mainly aquatic, and you will have to rise the water level, while still keeping some land.
Also do not keep them with the firebelly toads because I've seen what they do together at the pet shops. The poor newts are just crawling around minding their own business, and here comes a toad and grabs the little newt. That's why a lot of newts that are kept with them die, lose an eye or two, and lose limbs. On their own toads make great pets though, and 3-4 could be housed in a ten gallon, in an enclosure similar to the newts, or one with lots of moss and wood branches. It would have to have 1-2 inches of water still, and get misted 2-3 times a day.