About one liberty stamp :)
one
typicaly 2 stamps
Depends on how many pages the letter is; an average letter, say two pages - one stamp.
75 cents.
1
Your answer depends on the size and weight of the letter you want to mail. You can use the online calculator, below, from the USPS, to determine the value in stamps required to mail your letter.
only one
A single 45 cent First Class stamp is adequate for the delivery of a standard letter.
All you need is ONE first class postage stamp. Provided that it's only a letter.
One. From USPS web site: "Forever Stamps can be used to mail a one-ounce letter regardless of when the stamps are purchased or used and no matter how prices may change in the future.
If the letter weighs less than one ounce, then one 1st-class stamp will suffice. At present (August 2012), the postage rate is 45 cents.
One first class stamp will carry a normal letter anywhere in the US.