If you are hunting for a "new" Adobe Flash Professional CS6 serial number, you have likely run into a wall of dead links and "invalid" errors. Flash CS6 is a piece of legacy software released in 2012, and the landscape for owning it has changed drastically.
If you previously purchased a legitimate copy of CS6 and need to reinstall it, you can still use your original license: Purchasing a CS6 License - Adobe Community
The use of serial numbers and software activation served several purposes: