I probably should have said also many countries don't sell IRC's any more, so check yours does. (all countries must accept IRC's for exchange but don't have to sell them. So if you are sending to a country which doesn't sell them then that's not a problem).
If they don't then there are alternative methods such as paper currency/ stamps of the return country. Lots of topics on this here

You could look in to Global Forever Stamps which I will be using, sure you need 5 to send from the UK to US if it's more than 100g in weight but worth while if you get a response.

It really depends where your request is coming back from and the size/weight.

An IRC can be exchanged for stamps to the value of an airmail letter price up to 20g.

Australia is very generous and has one price band 0-50g so one IRC works.

Other places like the UK charge more if the item is 5mm+ thick, so even an item up to 20g but 5mm+ will require lots of IRC's.

I suggest you look at the various Postal Operators websites to check out their prices and so on. From there you can calculate how many IRC's you would need.

greenleaf07 wrote:Well, I was wondering because they said several so how mant is that several?

It depends on the size and weight of your envelope,how many ircs you need to include.Because one irc is the return postage for a standard envelope(c5) that weighs no more than 20g.So for example if you are sending something with 100g to be signed that you wouldhave to include 5-6 ircs.

Usually 3-4, it depends by the misure and weight of the envelope

Well, I was wondering because they said several so how mant is that several?