WPF string format

Text="{Binding StringFormat={}{0} items,Source={StaticResource mainVM},Path=Cnt}"

//xaml
<Window x:Class="WpfApp17.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        WindowState="Maximized"
        xmlns:local="clr-namespace:WpfApp17"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <local:MainVM x:Key="mainVM"/>
    </Window.Resources>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition Height="150"/>
        </Grid.RowDefinitions>
        <ListBox Grid.Row="0" 
                 ItemsSource="{StaticResource mainVM}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <Image Source="{Binding DataContext,RelativeSource={RelativeSource 
                        Mode=FindAncestor,AncestorType={x:Type ListBoxItem}}}"
                        Width="500"
                        Height="800"/>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
        <TextBlock Grid.Row="1" 
                   Text="{Binding StringFormat={}{0} items,Source={StaticResource mainVM},Path=Cnt}"
                   Panel.ZIndex="20" 
                   FontSize="100" 
                   Foreground="Red"/>
    </Grid>
</Window>



//cs
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Interop;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApp17
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            RenderOptions.ProcessRenderMode = RenderMode.SoftwareOnly;
        }
    }

    public class MainVM:ObservableCollection<string>,INotifyPropertyChanged
    {
        private int cnt;
        public int Cnt
        {
            get
            {
                return cnt;
            }
            set
            {
                if(value!=cnt)
                {
                    cnt = value;
                    
                }
            }
        }
        
        public MainVM()
        {
            var imgsList=new List<string>(Directory.GetFiles(@"../../Images"));
            if(imgsList!=null && imgsList.Any())
            {
                Cnt= imgsList.Count;
                foreach(var im in imgsList.Take(20))
                {
                    this.Add(im);
                }
            }
        }
    }
}