Go to /Application/Utilities and select Keychain Access.
Keychains are typically named, not numbered. You can see the keychains that are in your system by going View -> Show keychains.
A standard install of Mac OS X has a keychain set up for your account, named with your short-name.
Look for the "Internet Password" item (a blue @ symbol) with the title pop.mymailserver.net or whatever.
Is that what you wanted to know?