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.
4
2
I think you mean 9x12 envelope.
A 4-ounce large envelope currently costs $1.57.
It depends on how heavy the contents are and how big the envelope is.
I successfully mail CDs all the time with 2 stamps.