How many first class postage stamps are needed to send a letter from California to New York?

Depends on a few facts.

1.) The weight of the letter combined with the weight of the envelope!

2.) The shape of the envelope. 3.) The denomination of the stamps......i.e. $0.01 stamps, $0.05 stamps, $0.42, etc. (all USD). 4.) The chemical composition of your saliva (spit) for licking the envelope.

For example, if a #10 envelope weighs less than 0ne(1) once, the cost would be $0.42 USD. The number of stamps (you asked "How many") would be 42 if you used $0.01 stamps, all the way up to only 1 stamp if you used a $0.42 stamp

This site will explain everything from size to weights.

http://www.usps.com/welcome.htm

Good luck!