For a particular application, Subal recommends a "18/3" extension.

What do these numbers mean?

Also, I have an extension ring, how do I measure it to determine the extension?

Thanks in advance.

That is an 18mm long extension for a Type 3 mount.

The length is measured from the flat surface on the front of the female end to the flat surface at the base of the bayonet mount at the male end (ie length of extension not including the bayonet connection at the male end).