If Civil 3D 2020 is already installed and running, you can find your license info directly: : Type ABOUT and press Enter.
If you have the installation media (USB or download folder) but cannot log in, you can extract the product key from the internal files: Autodesk Product Keys - JTB World autodesk civil 3d 2020 serial number and product key
Are you using a license or a Commercial license? If Civil 3D 2020 is already installed and
: Open Civil 3D and click on the About command (often found in the top right-hand corner drop-down menu). Select Manage License and click the chevron icon to view the active serial number and license type. find "Civil 3D 2020".
: Under All Products and Services , find "Civil 3D 2020".