Start out with 3 to 4in. because it will settle. After it settles a couple of inches is usually enough. Sort of depends upon what you're using for mulch, too. You want it thick enough to both insulate the soil and keep the sun out so weed seeds don't germinate.
Be sure not to pile it up around the trunk of your little tree. Start the mulch an inch or two away from the base, and mulch outwards well past the reach of the branches. As it grows, extend the circle. Most trees have feeder roots out beyond their drip line.
