As many as you like really.I had these when I was young and their water needs changing reularly so it may be an idea to grow plants that can be attached to bogwood,slate or stone,that way you can lift them out and wash them under the tap when you clean the tank.
Also,your newts will need access to 'dry land' also,so you'll need at least a turtle dock for them,or if you can get a tall piece of wood or stone that sticks out of the water.I tend to suggest the same plants to everyone starting out,but it's because they are easy to grow and don't need mega lighting :java fern,java moss and anubias.
These all attach to hardscape for growing,each time my tank gets cleaned I lift them out and wash them under running water.
Look up 'paludariums' on the net to give you some good ideas for your critters home.
