WPF ItemsControl.AlternationIndex AlternationCount

<Style TargetType="{x:Type Control}" x:Key="lbxStyle">
    <Style.Triggers>
        <Trigger Property="ItemsControl.AlternationIndex" Value="0">
            <Setter Property="Background" Value="Red"/>
        </Trigger>
        <Trigger Property="ItemsControl.AlternationIndex" Value="1">
            <Setter Property="Background" Value="Green"/>
        </Trigger>
        <Trigger Property="ItemsControl.AlternationIndex" Value="2">
            <Setter Property="Background" Value="Blue"/>
        </Trigger>
        <Trigger Property="ItemsControl.AlternationIndex" Value="3">
            <Setter Property="Background" Value="Yellow"/>
        </Trigger>
        <Trigger Property="ItemsControl.AlternationIndex" Value="4">
            <Setter Property="Background" Value="Purple"/>
        </Trigger>
    </Style.Triggers>
</Style>

//xaml
<Window x:Class="WpfApp35.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:WpfApp35"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <Style TargetType="{x:Type Control}" x:Key="lbxStyle">
            <Style.Triggers>
                <Trigger Property="ItemsControl.AlternationIndex" Value="0">
                    <Setter Property="Background" Value="Red"/>
                </Trigger>
                <Trigger Property="ItemsControl.AlternationIndex" Value="1">
                    <Setter Property="Background" Value="Green"/>
                </Trigger>
                <Trigger Property="ItemsControl.AlternationIndex" Value="2">
                    <Setter Property="Background" Value="Blue"/>
                </Trigger>
                <Trigger Property="ItemsControl.AlternationIndex" Value="3">
                    <Setter Property="Background" Value="Yellow"/>
                </Trigger>
                <Trigger Property="ItemsControl.AlternationIndex" Value="4">
                    <Setter Property="Background" Value="Purple"/>
                </Trigger>
            </Style.Triggers>
        </Style>
        <Style TargetType="{x:Type TextBlock}" x:Key="tbkStyle">
            <Setter Property="Width" Value="300"/>
            <Setter Property="Height" Value="100"/>
            <Setter Property="FontSize" Value="30"/>
            <Setter Property="Foreground" Value="Black"/>
            <Setter Property="HorizontalAlignment" Value="Stretch"/>
            <Setter Property="VerticalAlignment" Value="Center"/>
            <Style.Triggers>
                <Trigger Property="IsMouseOver" Value="True">
                    <Setter Property="FontSize" Value="50"/>
                    <Setter Property="Foreground" Value="Red"/>
                    <Setter Property="FontWeight" Value="ExtraBold"/>
                </Trigger>
            </Style.Triggers>
        </Style>
    </Window.Resources>
    <Grid>
        <ListBox ItemsSource="{Binding BooksCollection,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
                 AlternationCount="5"
                 ItemContainerStyle="{StaticResource lbxStyle}"                 
                 VirtualizingPanel.IsContainerVirtualizable="True"
                 VirtualizingPanel.IsVirtualizing="True"
                 VirtualizingPanel.CacheLength="1"
                 VirtualizingPanel.CacheLengthUnit="Item">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel Orientation="Horizontal">
                        <Border Width="100" Height="100">
                            <Border.Background>
                                <ImageBrush ImageSource="{Binding ImgSource}" 
                                            Stretch="Uniform"
                                            RenderOptions.BitmapScalingMode="Fant"/>
                            </Border.Background>
                        </Border>
                        <TextBlock Text="{Binding Id}" Style="{StaticResource tbkStyle}"/>
                        <TextBlock Text="{Binding ISBN}" Style="{StaticResource tbkStyle}"/>
                    </StackPanel>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
    </Grid>
</Window>


//xaml.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.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApp35
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            var vm = new BookVM();
            this.DataContext = vm;
        }
    }

    public class BookVM : INotifyPropertyChanged
    {
        public BookVM()
        {
            InitData();
        }

        private void InitData()
        {
            var imgs = Directory.GetFiles(@"../../Images");
            if (imgs != null && imgs.Any())
            {
                imgsList = new List<string>(imgs);
                imgsCount = imgsList.Count;
                BooksCollection = new ObservableCollection<Book>();
                for (int i = 0; i < imgsCount; i++)
                {
                    BooksCollection.Add(new Book()
                    {
                        Id = i + 1,
                        ISBN = $"ISBN_{i + 1}",
                        ImgUrl = imgs[i],
                        ImgSource = GetImageSourceViaUrl(imgs[i])
                    });
                }
            }
        }

        private ImageSource GetImageSourceViaUrl(string imgUrl)
        {
            BitmapImage bmi = new BitmapImage();
            bmi.BeginInit();
            bmi.UriSource = new Uri(imgUrl, UriKind.RelativeOrAbsolute);
            bmi.EndInit();
            if (bmi.CanFreeze)
            {
                bmi.Freeze();
            }
            return bmi;
        }

        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged(string propertyName)
        {
            var handler = PropertyChanged;
            if (handler != null)
            {
                handler?.Invoke(this, new PropertyChangedEventArgs(propertyName));
            }
        }

        private ObservableCollection<Book> booksCollection;
        public ObservableCollection<Book> BooksCollection
        {
            get
            {
                return booksCollection;
            }
            set
            {
                if (value != booksCollection)
                {
                    booksCollection = value;
                    OnPropertyChanged(nameof(BooksCollection));
                }
            }
        }

        private List<string> imgsList { get; set; }
        private int imgsCount { get; set; }
    }

    public class Book
    {
        public int Id { get; set; }
        public string ISBN { get; set; }
        public string ImgUrl { get; set; }
        public ImageSource ImgSource { get; set; }
    }
}