You should look into your customer's license agreement. Enterprise Guide may be licensed for a given number of client PCs, but Web Report Studio does not have any client-side software, so you have no restrictions there. So look at their license for the server part, and I seriously doubt that there is a limit on the number of defined users. I know that our license doesn't have one; we're only limited by the number of cores we may legally use on the server.
How many users can use the WRS concurrently (and in a sensible way) is then mainly limited by the power of the server, and the data structure.
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