Runtime license files, must ALL developers working on a project embed their license files into a Solution?

Several Software Toolbox products require Embedded Runtime License Files which must be embedded within your Visual Studio.NET applications in order to remove any demo restrictions from whatever toolkit is in use.

Each developer who is licensed to use an applicable toolkit owns an embedded runtime license file of their own.

If you have a team of say 6 licensed developers working on the same Visual Studio.NET Solution,  then must all 6 of those developers embed their license within the same solution?

No.

Only 1 embedded runtime license is needed within the solution. As this question relates strictly to the Runtime of the application, it doesn't matter how many developers are working on the solution.

Now, the number of developers working on the solution at design-time does matter as far as licensing is concerned. Each developer who is using the applicable toolkit must be licensed to use it.