If you have lost your number and have proof of purchase, your best bet is to contact Horizon Hobby Support directly for a replacement.
Moving to Steam means you no longer need to keep track of a serial number for future installs—your license is tied directly to your Steam account , allowing you to install the sim on multiple PCs easily. RealFlight RC Flight Simulator 3. Activating and Registering Realflight 9.5 Serial Number
Please be aware that sharing your serial number publicly or using "cracked" serial numbers from unofficial sites can lead to your account being banned or expose your computer to security risks. Always use the official RealFlight Forums for technical support regarding lost keys. Elevator Trim - RealFlight Forums If you have lost your number and have