foreach (Course c in items)
{
    var lessons = context.Lessons.Where(l => l.CourseId == c.CourseId).Count();
}

<ListBox x:Name="listBoxLessons" Margin="10,47,10,10" VerticalContentAlignment="Top" HorizontalContentAlignment="Stretch" Background="{x:Null}" ScrollViewer.HorizontalScrollBarVisibility="Disabled" BorderBrush="#00ABADB3" VerticalAlignment="Top">
        <ListBox.ItemsPanel>
            <ItemsPanelTemplate>
                <UniformGrid Columns="4"/>
            </ItemsPanelTemplate>
        </ListBox.ItemsPanel>

        <ListBox.ItemTemplate>
            <DataTemplate>
                <Grid DockPanel.Dock="Top" Height="200" MaxHeight="200">
                    <Grid.RowDefinitions>
                        <RowDefinition Height="10*" MaxHeight="50"/>
                        <RowDefinition Height="30*" MaxHeight="60"/>
                        <RowDefinition Height="30*" MaxHeight="20"/>
                        <RowDefinition Height="50*" MaxHeight="70"/>
                    </Grid.RowDefinitions>
                    <Border BorderThickness="0" BorderBrush="Red" Background="#FFF7F7F7" CornerRadius="5,5,0,0" Grid.Row="0">
                        <Label Content="{Binding Name}" VerticalContentAlignment="Center" FontSize="18" FontWeight="Bold" HorizontalContentAlignment="Center"/>
                    </Border>
                    <Label x:Name="labelLessonAmount" Content="{Binding AmountLesson}" Grid.Row="1" VerticalContentAlignment="Center" FontSize="14" Background="#F7F7F7" VerticalAlignment="Stretch" HorizontalContentAlignment="Center" />
                    <ProgressBar x:Name="progressBar" Grid.Row="2" Value="20" />
                    <Button x:Name="btnStartCourse" Content="Start" BorderThickness="0" Style="{DynamicResource ButtonStyleListViewBox}" Grid.Row="3" Command="{x:Static commands:CourseOverviewWindowCommands.StartCourse}" CommandParameter="{Binding}" />
                </Grid>
            </DataTemplate>
        </ListBox.ItemTemplate>

foreach (Course c in items)
{
    course.AmountLesson = context.Lessons.Where(l => l.CourseId == c.CourseId).Count();
}