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 * radius^{2} * length = volume
3.14 * 6^{2} * (60 * 12) = 81,430
I have a 2x4foot 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 "20pound stock" or "24pound 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:
