I just bought and installed Acrobat XI Pro and wanted to register it. I got a message that my serial key has been used for more than the required number of systems

You were almost certainly scammed. Adobe no longer sell the obsolete Acrobat XI but pirates do. Get a refund.

using a browser that allows popups and cookies, contact adobe support during pst business hours by clicking here and, when available, click 'still need help', https://helpx.adobe.com/contact.html and request an activation count reset.