In order to get a useful answer, you're going to have to provide a better idea of what kind of app you're talking about. To use a couple of extreme examples, i

In order to get a useful answer, you're going to have to provide a better idea of what kind of app you're talking about. To use a couple of extreme examples, if your app provides key-cracking services, and each user wants to crack 1 key per day, and each key takes about a server-day to crack, you'll need (duh) at least 1 server per user. On the other hand, if your app is basically a blog, a single (beefy) server should be able to quite easily serve millions of readers a month with the right caching in place.