It is probably hard to get an exact number, but here is how I would start answer to this question: First, we have to define what a "tree" is. Is it a giant redwood tree or a little weeping willow? Most paper is made from pine trees, so I went out in the woods and looked at some pines.
Most are about 1 foot in diameter and 60 feet tall. Ignoring taper, that's about 81,430 cubic inches of wood:
pi * radius2 * length = volume
3.14 * 62 * (60 * 12) = 81,430
I have a 2x4-foot piece of lumber in the backyard. It weighs about 10 pounds and contains 504 cubic inches of wood. That means a pine tree weighs roughly 1,610 pounds (81430/504 * 10).
I know that in manufacturing paper, the wood is turned into pulp. The yield is about 50 percent -- about half of the tree is knots, lignin and other stuff that is no good for paper. So that means a pine tree yields about 805 pounds of paper. I have a ream of paper for a photocopier here and it weighs about 5 pounds and contains 500 sheets (you often see paper described as "20-pound stock" or "24-pound stock" -- that is the weight of 500 sheets of 17" x 22" paper). So, using these measurements, a tree would produce (805/5 * 500) 80,500 sheets of paper.
These are all fairly rough estimations, and I weighed things on a bathroom scale, but you get the general idea.
Here are some interesting links: