To get "a" number. Using Firefox.
Log in. Go to the bottom of the home page and it will show you you are online. Hover the mouse over your name and at the bottom left of the screen it will show a link. In this link is a number and this is your unique database membership number. It is not a professional membership number nor anything except how you are referenced on the forum database. It, technically, is your Yamaha Owners Club unique number.
I would use it with caution however, what is the difference between any other club and this one (It has club in the title) and you have paid your £10 so can show you are a "Professional" member of the club. If I started a club, got some cheap cards printed and wrote a membership number on it, would this be any different. Just make sure that they are not looking for any affiliations to a "recognised" club like, as mentioned, the BMF.