That will depend on the value of the stamps and perhaps the dimensions of the envelope. If you have the right values, it would only be one, but it could be a dozen or so if you only have smaller values. Check your Post Office web site for the specific rate necessary.
I think you mean 9x12 envelope.
A 4-ounce large envelope currently costs $1.57.
Two stamps
That depends on where you are sending it from (you country of origin) and to (the country of destination).
how many stamps for a 10.4 ounce envelope
The first ounce will cost 44 cents. Each additional ounce, or fraction, is another 17 ounces. That would be 78 cents for that envelope at the start of 2011.
Take your envelope to the Post Office and they will weight it, and give you your answer as the cost of mailing anything is done by weight.
The amount of stamps needed for an envelope is determined by weight. Weigh your a4 envelope and you will know how much postage is needed.
how many stamps for a 3.4oz envelope
You do not need airmail stamps because all letters go by airmail. The postage for a standard sized envelope weighing one ounce or less is 98 cents. There is a postage calculator at http://ircalc.usps.gov/
u need to use usa stamps as English ones are not valied on a letter from America to England ============================================== Postage from the USA to England is 90 cents for a regular letter.
That depends what denomination of stamps you place on the envelope.