Q A - Xamarin.Form: Don’t want to target one or more platform at a same time?

Create New Xamarin.Form solution after that clear unwanted target project. If you required to add project on feature.

You can add new platform projects to the Xamarin.Forms solution.

 In the Add New Project dialog, you can create Xamarin.iOS project by selecting the iOS project Universal type and Blank App template. 

Create a Xamarin.Android project with the Android Blank App template, or a Windows project by selecting Universal under the Windows heading (for a UWP project), or Windows or Windows Phone under the Windows 8 heading, and then Blank App.