You can place any number of classes in a single file as long as it compiles OK. However, remember that when you build a page, control or web service each can only be derived from a single class.
If you place several classes in a single file when they are compiled (either manually by you or automatically by the ASP.NET worker process) all the classes are accessible and usable by the code in them. In other words, if you build a utility class in the same file as an ASPX code-behind class, the code-behind class can access that utility class.
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Peter