If you are sending a standard size envelope, then 1 IRC is apt for for return postage from the UK to anywhere in the world. Any envelope bigger than that, I would suggest 2 IRC's or more - but as guilder1st said, it really would help if you mentioned the envelope's size.

It helps if you tell us what country you are located in, and what size the envelop is you are sending(Yellow envelopes come in all different sizes). To make sure of how many stamps to use, I'd say go to your local post office they can give you exact weight and price.
For IRC's? Assuming you are in the USA you will need to include at least 2-3 IRC's for return postage. {thumb2}

Im sending my autograph request in a 10 by 13 envelope and the retrurn envelope is a 9 by 12? Im sending it to the UK from the USA how many stamps do I need to put on it? And is 1 IRC enough for the return postage?