As of February 2015, a 1-ounce letter from the United States to Verona, Italy costs $1.15 to mail and would require three stamps. It could also be done with one stamp if the sender is willing to overpay by using the available $1.20 denomination.
The three stamps that would meet the exact amount are the $1, 10 cent and 5 cent denominations, but the sender could use any other available combination. Restrictions and limitations as well as available stamp denominations can be found at the USPS website, which offers a postage calculator. Another option is to go to any post office and allow the clerk to weigh the letter; the clerk can then apply a postage-paid label to the letter upon receiving an exact payment.