It's impossible to say how many as it can be a few hundred or a few thousand. It's always better to have many people report the same site than just one person doing it, because that increases the priority to have the authorities or companies such as Google actually look into it.

Based on my own experience, it varies a lot. I have reported numerous scam applications in the Google Play Store, where some are being removed in a day, while others are left active for weeks. I think it yet again comes down to how many people report them. The more people report an application, the more likely it is that something is actually wrong with it, so Google has to act swiftly.

What I do nowadays is that I ask friends and family to report it as well, which increases the odds of a quick removal.