How to add mobile no filed in contact us form.
You can easily add a mobile number field in the contact us form by modifying the view file:Views\Common\ContactUs.cshtml
But it you will also need to modify nopCommerce sources by changing the ContactUsModel and the CommonController logic in the ContactUsSend action method as well.
You need to have some basic knowledge of MVC and C# to do that.
Hope this helps!