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How many stamps would I need if I was sending a letter
from Canada to the USA ?

This is probably in the wrong section.

$0.93 for a standard letter up to 30g
$1.86 for oversized letter up to 100g

The obsolute best way if shipping to another country is to get your letter all done and sealed bring it to the post office let them weigh it and they'll tell you the amount of postage required.