One first class postage.
Only one
The number of stamps that you need to send a letter from Australia to London, will depend with the value of one stamp.
one
1. it's the same for all of the US
3 stamps. An average stamp is $0.44 so one would need $1.32 to send a letter from London to Utah.
1 oz letter to UK is .94
Nine
$1.05 = Forever stamps are worth 45 cents each. so a little more than two of these stamps
One stamp will cover one ounce of first class mail to anywhere in the USA.
One, if it is the correct USA amount; see the Universal Postal Union of 1949.
A single 45 cent First Class stamp did the job after 22 January 2012.
TWO US Forever stamps will suffice (USA to London post card)