It can vary. Sometimes it can take only one. Other times it can take more. It's not always good to mass report, though. It's more likely to be taken down if it actually breaks the guidelines.

I think it doesn't matter how many reports people put on your project. Scratch Team will or will not evaluate this reported project.

If the project gets reported because people hate it, or are jealous that it was so popular, Scratch Team will NOT care if it is appropriate or not. They'll just report it.

If the project gets reported because people think it's not appropriate, Scratch Team will evaluate it and decide whether it gets to stay (Which is highly UNLIKELY) or whether it deserves to be taken off the website (Which is most likely to happen).

Hope this answer helps!