Autodesk has largely phased out serial numbers for many single-user subscriptions in favor of sign-in activation (Autodesk ID). However, if your license requires one, you can find it using these methods: View your product license information - Autodesk
To launch and use Civil 3D 2020 legally, follow the authorized activation steps outlined by Autodesk Support: Method 1: The Sign-In Method (Recommended) Launch on your computer. autocad civil 3d 2020 serial number and product key
Instead, let’s get you the right information so you can get back to designing roads, grading sites, and modeling corridors. Autodesk has largely phased out serial numbers for