* Multiple Shows: There's the original "Scooby-Doo, Where Are You!", "Scooby-Doo on Zombie Island," "What's New Scooby-Doo," "Scooby-Doo! Mystery Incorporated" and many, many more spin-offs. Each one features numerous episodes.
* Different Canons: The shows don't always exist in a single, consistent universe, so some mysteries might be considered canon in one show, but not in another.
* Recurring Villains: Some villains like Ghostface, the Phantom, and Captain Caveman appear in multiple shows, which makes it hard to know if they've been "solved" once or several times.
In Conclusion:
Instead of trying to count all the mysteries, it's more fun to appreciate the Scooby-Doo universe for its vast and enduring legacy of spooky shenanigans!