newts will generally need access to land and access to water, some species can't swim as well, others swim very well, so it probably comes down to your preference in care, and of course the size tank. So, for instance, tiger salamanders are the largest land salamander
so a single adult would need something like a 10-20 gallon tank alone. They also don't swim so they need a big shallow dish of water.
But something like a firebelly newt
is smaller, so 3-4 is a good amount for a 20 gallon. These guys can swim, and they make use of land and water.