This information is now out of date as ZBrush has moved to subscription only. Your subscriptions should be managed through the Maxon App and is no longer linked to Pixologic. For up-to date information please
see here.
The ZBrush serial number is not stored anywhere within your ZBrush interface.
If you no longer have the email that was sent to you containing your serial number you can retrieve it at any time by going to https://pixologic.com/my-licenses/login.php
This web page will require you to log in using the same user name and password that you use to log into Pixologic Support. Once logged in you will be able to view all serial numbers associated with the registered email address.
If you have any licenses associated with a different email address, those will not be listed. You would need to either create a Support account under the other email address or submit a Support ticket with Pixologic to request that your accounts be merged.