To find out your FAN number you can head to https://secure.thefa.com/fan/retrievefandetails.aspx and fill out the details on the page.
You should then receive an email which will tell you the FANumber attached to that email. If there is more than one number you need to tell your local county FA that you require a ‘de-duplication’ or a ‘merge’. You should include your FANs in your email to that county. This is important as it will allow you to log in to Matchday but it’ll also ensure you keep any coaching/safeguarding/referee qualifications in one place.
You can find any county's contact details by searching [County] FA into your search engine, navigating to 'about' then to 'contact'.