How many International Reply Coupons should I include?

Hi, I live in Canada and i would like to write a letter (normal, no more than 1 oz.) to New York. I would like to include International Reply Coupons (purchased in Canada for 3.50(CAD) each) so that I can receive a letter back and they dont have to pay at all they just use the IRCs.. how many IRCs should i send them?