Hopefully a relativity simple question. I'm starting work on a new internal project to create tractability of repaired devices within the buildings.

The database is stored remotely on a webserver, and will be accessed via web API (JSON output) and protected with OAuth. The front end GUI is being done in WPF, and the business code in C#.

From this, I see the different layers Presentation/Application/Datastore. There will be code for managing all the authenticated calls to the API, class to represent entities (business objects), classes to construct the entities (business objects), parts for WPF GUI, parts of the WPF viewmodels, and so on.

Is it best to create this in a single project, or split them into individual projects?

In my heart I say it should be multiple projects. I have done it both ways previously, and found testing to be easier with a single project solution, however with multiple projects then recursive dependencies can crop up. Especially when classes have interfaces to make it easier to test, I've found things can become awkward.