If you are using a legacy version (e.g., 5.x or 6.x) or require an offline setup, here is how you can find and apply your license details: Locating Your License Information
Create a free RealVNC account. You get:
Searching for a “VNC Server license key serial number” is a trap. Not only are most results outdated or fraudulent, but the few that offer cracks expose you to serious security risks. RealVNC’s activation systems have rendered keygens obsolete for modern versions. Vnc Server License Key Serial Number