Hi,
Web Services are the underpinning of Microsoft's .NET strategy. The concepts and the innovations behind this initiative have struck a chord with developer's building the next generation of Internet applications. A Web Service is programmable application logic accessible via standard Web protocols.
So, a web service can work not only for n number of users also it can work in n numbers of web-sites.
You can have only one web service in your condition.
Refer
here[
^] for more information.
--Amit