You should probably go to your Postal Office, because it depends on the letter weight, what is in it, and how fast you want it to be shipped.
12 stamps
It depends entirely on the value of the stamps. In 2010, a standard letter currently requires a stamp or stamps to the value of 60c within Australia.
The number of stamps that you need to send a letter from Australia to London, will depend with the value of one stamp.
3 stamps.
typicaly 2 stamps
You need 1 stamp to mail a letter/package throughout Canada
10
Currently as of 2011, it costs 59 cents.
1 stamp
500 if your in America/Canada
As of October 2011, Canada Post domestic lettermail costs $0.59
98 cents worth