How can I display an itemsCount for Expander content - wpf

Question

I have spent wayyyy too much time on this, so if someone can help point me in the right direction, I would be very appreciative.
I have an Expander with a ListBox in it. I have bound the ListBox to a List ie FilteredProjects. I would like the Header of the expander to reflect the number of items in the displayed list. I did implement INotifyPropertyChanged for ProjectsCount, and the ItemsSource for the ListBox updates just fine. Thanks in advance for your time.
<Expander>
<Expander.HeaderTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<TextBlock Text="Projects" />
<Border CornerRadius="4" Padding="0" BorderThickness="1" BorderBrush="Black" Background="#FF545F42" Margin="1,0,0,0" >
<Label Content="{Binding Path=ProjectsCount, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource filter}}" Padding="0" FontSize="8" FontStyle="Italic"></Label>
</Border>
</StackPanel>
</DataTemplate>
</Expander.HeaderTemplate>
<ListBox x:Name="ProjectsFilterListView" Opacity="1" ItemsSource="{Binding FilteredProjects}" >
<ListBox.ItemTemplate>
<DataTemplate>
<CheckBox IsChecked="{Binding IsSelected}" Content="{Binding ItemName}" Margin="10,10,0,10"></CheckBox>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
</Expander>

Answer 1

You could just get the Item Count directly from the ListBox
<Label Content="{Binding Path=Items.Count, ElementName=ProjectsFilterListView}" />
Here is an example of it working, I replaced Label for TextBlock so I could use StringFormat to add "Items:" text before the number as I am not sure what your Converter is doing
<StackPanel Orientation="Horizontal">
<TextBlock Text="Projects" />
<Border CornerRadius="4" Padding="0" BorderThickness="1" BorderBrush="Black" Background="Red" Margin="1,0,0,0" >
<TextBlock Text="{Binding Path=Items.Count, ElementName=ProjectsFilterListView, StringFormat={} Items: {0}}" Padding="0" FontSize="10" FontStyle="Italic" Foreground="Black"/>
</Border>
</StackPanel>
Result: