One project for all devices
When creating Xamarin solution, I suggest not to add a project for each device (one for ios, another for Android…). It will be easier to have one project that contains classes for common features and classes for each device. This can be done by some naming conventions, using separate folders for each OS, or by marking each class by some attributes (like [ios], [android]… etc.). It's easy then to compile this project to have many apps, each contains only the classes that suits the device it will work on.