WPF DataGrid / ListView bind to array mvvm

Ok, so if you're going to do this properly, you'll first need to create a data type/model class to hold your data. It should implement the INotifyPropertyChanged interface correctly and contain a property for each column that you want to display in the DataGrid including an extra one for your 'Q2' requirement.

Next, you want to add two properties of type ObservableCollection<YourDataType> into your code behind/view model. The first will hold the whole collection and the second will display just the number of rows that you want. You'll also need an integer property to Bind to the selected item of the ComboBox:

<DataGrid ItemsSource="{Binding FilteredItems}" ... />
...
<ComboBox ItemsSource="{Binding Numbers}" SelectedItem="{Binding SelectedItem}" />

Now, whenever the SelectedItem property changes in the view model, you just need to update the number of rows of items in the FilteredItems property:

public int SelectedItem
{
    get { return selectedItem; }
    set
    {
        selectedItem = value;
        NotifyPropertyChanged("SelectedItem");
        UpdateFilteredItems();
    }
}
...
private void UpdateFilteredItems()
{
    FilteredItems = 
        new ObservableCollection<YourDataType>(Items.Take(SelectedItem));
}

In the UpdateFilteredItems method, we simply take the relevant number of items from the whole Items collection based on the SelectedItem value. As the FilteredItems collection is bound to the DataGrid.ItemsSource, the UI will automatically update.