Your membership number can be found, together with affiliate badges and banners for your website, on your affiliate page.
On that page, scroll down to the highlighted yellow bar, which shows your direct affiliate link.
Your membership number is the three or four digit ID at the end of the link, after “affid=”
http://allianceindependentauthors.org/?affid=[your membership number]
If you have any problems finding your number, email Philip: email@example.com