<GroupStyle>
    <GroupStyle.HeaderTemplate>
        <DataTemplate>
             <TextBlock Text='{Binding Key}' Foreground="{StaticResource ApplicationForegroundThemeBrush}" Margin="5" FontSize="18" FontFamily="Segoe UI" FontWeight="Light" />
        DataTemplate>
    </GroupStyle.HeaderTemplate>
 </GroupStyle>

public List<ItemList> CreateGroupedData()
{
    if (ReceivedList!= null)
    {
        var result =
        from t in ReceivedList
        group t by t.GroupField into g
        orderby g.Key
        select new { Key = g.Key, Items = g };

        List<ItemList> lists = new List<ItemList>();
        foreach (var i in result)
        {
            ItemList list = new ItemList();
            list.Key = $"{i.Key.ToString)} [{i.Items.Count.ToString()}]";
            lists.Add(list);
        }
        return lists;
    }
}