Does anyone know?

Yes, that is your key id -- you'll need it in case you need a replacement. Write down the number somewhere.

My dealer told me not to keep it on the keychain with the key I use all the time bcause 1) if you loose the key you've also lost the tag needed to get another one made and 2) if someone gets hold of your keys they could make copies.

As I understand it the only way to get an extra key made is from Mazda due to the interlock that's built in and unique. The number on the tag is the only way to identify a lost key and have one remade.

Maybe I'm a little obsessive, but I wrote down the number and put it in my RX-8 file, and then put the metal plate in our safe deposit box at the bank along with the rest of our household papers.

That's not obsessive at all. Prudent would be the term.