For Vendors:
Before discussing keys, let's clarify the product. VNC Server is the software installed on a remote machine that you want to control. For personal use (connecting to your own home PC from your laptop), the of VNC Connect (RealVNC) or UltraVNC is usually sufficient.
Is there a specific VNC product you're trying to license? I can point you to the official source or a free alternative.
If you need VNC capabilities but cannot afford a commercial license, you have better options than searching for a cracked serial number:
: This is the standard for most plans. You activate the server by signing in with your RealVNC account credentials (email and password). Offline Licensing : Exclusive to Enterprise