Hi Gingga!
Firstly, welcome to Fanmail.biz! {clap} I hope you enjoy it here!!
I live in Thailand and I want to send a letter to my favourite star but I don't know how many IRCs I must send to recieve my letter back?
I recommend sending
at least 2 IRC's, although if you want to be safe, you should rather send 3.

Lately I've seen people sending 4 IRC's! Also, it is considered polite to pay for the return postage as you want something back from the celebrity! {thumb2}
I have to put another envelope into the outer envelope that I send right?
Correct. On that envelope, you write your own mailing address. The celebrity will use that envelope to mail you a signed photo back.

You can read more about SAE, SASE and SSAE
here.
Is IRCs according to the weight I send?
As Melindabear2002 said, the US work with both weight and size. But as far as I know, other countries work with size.
If you have any more questions, please feel free to ask!
Good luck collecting,
Lizanne