Just one. It has to be worth 42 cents. That is the rate for first class mail anywhere in the U.S.
You can also use any combination of stamps that add up to 42 cents. Stamps come in many...
Unless the paper is very thin/light, it takes 2 stamps. 1 stamp can usually cover about 6 pages. It depends on the type of postage, the weight, where it is going and the type of transport