One strong tree is enough, but building a tree house in a single tree tends to be more difficult than in many. It’s easiest with 2, 3 or more trees spaced 2 – 6 m (6–20 ft) apart, as you can attach your platform beams nicely and get a large platform area.
If the trees are too small, you can of course build on stilts or choose a combination of tree attachment and stilts.