How do you know that your Serial Number is registered?

There is a complicated method that involves Terminal commands and is very detailed. That method, I am not familiar with but there are Others who are and may offer to assist.

Then there are two other methods which may serve your purposes.

1 - Revert your Mac to a previous macOS version and read the details and print out the steps before attempting.

2 - What to do before you sell, give away, or trade in your Mac. Realize you are not selling or giving any but the method will work in your specific situation. NOTE: all personal files, settings, data, applications will be remove and will have to be reinstalled.

Welcome and Thank you too :-))

The Serial Number of any Apple Device is unique and can not be duplicated.

As for " how many times can a Serial Number may be registered? " as in registered where ?? If you mean Registered and Associated to ONLY ONE Apple ID - once.

If you mean registered to More than One Apple ID - guess it is possible to have the same device with the Unique Serial Number associated with another Apple ID - though I have never had more than one Apple ID

The Serial Number of the device has no bearing on having an email account on whoever one uses for a e-mail server.

Use wrote " once. I had an Apple ID, which was supposed to be changed into a normal Apple ID, " There is not such thing as a Normal Apple. An Apple ID is an account with Apple for several purposes like purchases of Apple Devices, Music, Apps for iPhone etc and if one chooses to use iCloud for that too. iCloud services include the usage of iCloud and iCloud e-mall address.

But you already know about Apple ID as you logged into your Apple Account earlier to find your devices serial number.

Can speak to your comment this way. There are basically two types of compute Ownerships. That which is owned by a Company or Organization and Private Ownership. The first is where the company may issue a computer to an employee to be used for business needs. They may also enroll the computer in a MDM - Mobile Digital Management Program. Then the company's IT Depart decides how the computer is used including what can be or can not be installed. There is nothing the employee can do about that.

Privately Owned computer - the person who paid for the computer becomes the de facto MDM Service. They, parent, uncle, cousin etc. is the Admin of the computer and has total control again , how it is used and what can be or can not be installed on the machine. If the Admin decides to allow additional accounts on the machine, they decide what level of control this account has or does not have and may include Parental Controls enable. Then there is Guest Account and that account has zero control over the computer and as soon as they leave the Guest Account - whatever was done automatically is deleted. .

Now, I sense there might be a moral question of control of the computer. That is out of my abilities to comment or preview to directly comment on this. Other than to speak to the Owner of the computer and explain your point of view.

Log into https://appleid.apple.com/ using your Apple ID and password. All devices associated to your user id will appear including Serial Numbers of each device.

Serial number of what?

Thank you too and stay safe and well :-)