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I think you mean 9x12 envelope.
how much postage for a 9x12 envelope weighing 6.6 ounces
how much postage required for a 9x12 envelope weighing 4.6 ounces
$3.04 as of 2016.
$3.04 as of 2016.
Those dimensions make it a large envelope, so postage starts at 94 cents (two Forever stamps) for the first ounce, and more if it's heavier.
Postage depends on which country you live in, how much the envelope and contents weigh, and where you are sending the envelope to. You'll need to include all that in your question to get the correct answer - or you could just go to your local postal website and get the answer from the experts...
.92
Calculate it on usps.com
At least two for those dimensions, possibly more depending on weight.
9x12 = 108
9x12 9x12