How do I know if I have more than one Apple ID account.

You either remember them or you don’t. You can try and look it up (If you forgot your Apple ID email address or phone number - Apple Support) but only you would know what email addresses or (in limited countries) telephone numbers you’ve used to create an AppleID.

You cannot cleanly combine any AppleIDs as purchases cannot be merged. And you can only delete an AppleID if you know it and can login it. Being able to login to an AppleID is a necessary requirement to prove ownership and control of it in order to request it be deleted -> How to delete your Apple ID account - Apple Support