I have mostly 20 gallon longs (30x12inches) and keep them with 12-15 gallons of water. It really depends on the setup and filtration. Surface area is also key. you can have 50 gallons, in a tall tank with wasted space. How long and wide is your setup? If you have a setup with thriving plants and good water quality, you keep 6 in a 30x12 inch tank very comfortably. If your surface area is 12x36 you could house upto 8 comfortably. I keep 8 very large Spanish ribbed newts in a 48x12 inch tank (50 gallon cut down to 33gallon). They are fine with weekly water changes, good filtration, and tons of flourishing plants. Ribbed newts are your worst offenders of fouling up a tank (big eaters and poopers!). Cynops are smaller bodied newts and easier to manage.
I think the best way to maximize your "surface" area (underwater and land) is try Jen's idea with an island approach. You have 1/3 of surface for land created by an island (bricks and 12 inch plant saucers) The newts have the full tank surface area underneath while an island for land. I use this for my axolotls as well. Instead of a land area (above water), I set up shelves under water increasing their surface area below. This give them hiding spaces and keeps them off of one another.
hope this helps.