Each IRC is worth the minimum value of an airmal letter. For example, in the UK the cheapest Airmail letter rate is £0.88. So one IRC in the UK is worth £0.88 of postage.
To know whether one IRC will be enough, you need to weigh your return item (or make the best guess possilbe) and then research airmail prices on the various countries websites.
They nearly all have calculators to help you find the information you need easily.
I see from your previous posts you've used IRC's already to the UK & USA so you should be fairly familiar with how it works.