How many stamp are needed to send a letter from the US to Pennsylvania?

The First Class US postage rates (not including Priority Mail) are the same to and from anywhere in the US.

For a one ounce regular-size letter mailed in May 2009, you need 44 cents in stamps or one Forever Stamp. Additional ounces are 17 cents for letters up to 3.5 ounces.

Note: The rates change every year. The above rates were current when this answer was written.