My Xaml

<Window x:Class="Window016"
         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"
         xmlns:local="clr-namespace:WpfApp1.WpfApp016"
         xmlns:mod="clr-namespace:WpfApp1.WpfApp016"
         mc:Ignorable="d"
         Title="Window016" Height="450" Width="800">
   <Window.Resources>
      <local:ViewModel x:Key="vm"/>
    <Style x:Key="HighlightingTextStyle" TargetType="Label">
       <Setter Property="FontStyle" Value="Italic"/>
     </Style>
   </Window.Resources>
   <StatusBar>
        <Label Content="Selected Devices:" Margin="1,1,0,1"/>
        <Label Content="{Binding SelectedDevicesCount, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
    </StatusBar>
   <Grid DataContext="{StaticResource vm}">
     <TreeView ItemsSource="{Binding TreeViewDataCollection}">
       <TreeView.Resources>
         <HierarchicalDataTemplate ItemsSource="{Binding HostGroupName}" DataType="{x:Type mod:RootNameList}">
           <StackPanel Orientation="Horizontal">
             <Label Style="{StaticResource HighlightingTextStyle}" Content="{Binding RootSubnet}"/>
           </StackPanel>
           </HierarchicalDataTemplate>
         <HierarchicalDataTemplate ItemsSource="{Binding Host}" DataType="{x:Type mod:HostGroup}">
           <StackPanel Orientation="Horizontal">
             <CheckBox IsChecked="{Binding IsParentChecked}" IsThreeState="True"/>
             <Label Style="{StaticResource HighlightingTextStyle}" Content="{Binding GroupName}" Tag="{Binding RelativeSource={RelativeSource AncestorType=Window}}"/>
           </StackPanel>
           </HierarchicalDataTemplate>
         <HierarchicalDataTemplate DataType="{x:Type mod:HostName}">
           <StackPanel Orientation="Horizontal">
             <CheckBox IsChecked="{Binding IsChildChecked}"/>
             <Label Style="{StaticResource HighlightingTextStyle}" Content="{Binding Host}" Tag="{Binding RelativeSource={RelativeSource AncestorType=Window}}"/>
           </StackPanel>
         </HierarchicalDataTemplate>
       </TreeView.Resources>
     </TreeView>
   </Grid>
 </Window>
 
 
 My ViewModel
 
 public class VM : BaseViewModel
 {
 
		internal void SelectedDevicesCounter(int count)
        {
            SelectedDevicesCount = count;
        }
 
	private int _selectedDevicesCount;
        public int SelectedDevicesCount
        {
            get
            {
                return this._selectedDevicesCount;
            }
            set
            {
                this._selectedDevicesCount = value;
                OnPropertyChanged("SelectedDevicesCount");
            }
        }
 }
 
 
 My Model
 
 public class HostName : BaseViewModel
    {
        VM ncmvm = new VM();

        
       

        public string Host { get; set; }
        public string HostType { get; set; }

        private bool? _isChildChecked = false;
        public bool? IsChildChecked
        {
            get
            {
                return this._isChildChecked;
            }
            set
            {
                this._isChildChecked = value;
                if(value == true)
                {
                    Constants.counter += 1;                   
                }
                else
                {
                    Constants.counter -= 1;
                }               
                OnPropertyChanged("IsChildChecked");
                ncmvm.SelectedDevicesCounter(Constants.counter);
            }
        }

        public override string ToString()
        {
            return Host;
        }
    }