how to write a moq test to setup a method to return a count value

In my controller action, whenever a new product is added I check in the database that this product no is not present already. The code for this check looks like

 public ActionResult Index(ProductModel model)
    {
      var productCount = _productsService.GetAll(true).Count(x => x.ProductNumber == model.ProductNumber);

      if (productCount > 0)
          ModelState.AddModelError("ProductNumber", Product already present in the system!");

       // more processing
     }

I m new to MOQ testing and trying to write a unit test to setup the GetAll method which will return 0. I have written something like this but it does not seem to work

 var _productsService = new Mock<IProductsService>();
_productsService.Setup(m => m.GetAll(true).Count()).Returns(0);

Any ideas? thanks