How many licences do I need for a web-based application that runs on one server but can be accessed by many clients?

A single user licence is sufficient for an application running on a single server. If the control is being used clientside, e.g., by using Javascript in a web page, the licence information can be downloaded from the server when needed. This page explains how to use a licence package file (.lpk) to access licence information on the server.

If you are maintaining a separate test environment for your application, in addition to the main server, you may need a 2 user licence instead.