Most times, I've seen a fanmade take of Super Mario Bros. that have 8 Worlds minimum or lower if the project is smaller, sometimes, with a secret world, but have you had an idea where you thought of having >9 Worlds in your game?

Well, most real Mario games have eight worlds (and possibly a bonus world). It's certainly doable (SMB X is a good example), but it would be a mighty ambitious project unless you had a lot of people helping.

well if you're going to make a long game you gotta keep people interested- otherwise things are just going to get repetitive and people are just going to wonder why they should even bother continuing. deciding when a game should end is a pretty important part of making a good game i think- and since mario games have an established length it's not something most people feel the need to worry about

One of my other games I plan to make S's Levels which will be on HE5 will have more than nine worlds.