It only takes a single stamp. But it does have to be the properdenomination. Currently the rate for a one ounce flat in the US is 88 cents. (10/2009)
what is the cost to mail an A9 Announcement envelope 6x9
how many stamps for a 3.4oz envelope
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.
That depends what denomination of stamps you place on the envelope.
As many as will fit. I have one envelope in my collection with 44 stamps on it! The sender used one cent stamps to mail it. Some people will overlap stamps to get more to fit, which is okay as long as the Post Office can plainly see that they are all there.
two
It depends on tne weight of the envelope not the size.
2
4
A 4-ounce large envelope currently costs $1.57.
I think you mean 9x12 envelope.
It depends on how heavy the contents are and how big the envelope is.