That depends. I tend to leave small support classes in the same file, but anything public goes to file per type.
Why? Because I have to work with people who don't have resharper installed, and without resharper it's near impossible to find those types without doing a full project search.
If you're talking about a project with thousands of classes in a similar number of files, with auto-generated code from an ORM as part of it. Yes.
Search takes at least a minute. Class view is only useful if you know the namespace. Go To Definition doesn't help if you're looking a definition for an interface and you want the concrete implementation.
Why? Because I have to work with people who don't have resharper installed, and without resharper it's near impossible to find those types without doing a full project search.