How to display the RowIndex in GridRowHeaderCell when combined with SfDatapager in WPF DataGrid(SfDataGrid) ?

You can display row numbers in the GridRowHeaderCell by customizing the RowHeaderCell style. This can be achieved by binding the RowIndex to the TextBlock.Text property and using a MultiValueConverter. Additionally, you can handle the PageIndexChanged event in SfDataPager. This is demonstrated below.

<Style TargetType="syncfusion:GridRowHeaderCell">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="syncfusion:GridRowHeaderCell">
                <Border x:Name="PART_RowHeaderCellBorder"
                        Background="Blue"
                        BorderBrush="{TemplateBinding BorderBrush}"
                        BorderThickness="{TemplateBinding BorderThickness}">
                    <Grid>
                        <!--//RowIndex is displayed here-->
                        <TextBlock HorizontalAlignment="Center"
                                   VerticalAlignment="Center"       
                                   Foreground="White"                                   
                                   TextAlignment="Center"   >
                            <TextBlock.Text>
                                <MultiBinding Converter="{StaticResource rowIndexConverter}" >
                                    <Binding  Path="RowIndex" RelativeSource="{RelativeSource TemplatedParent}"/>
                                    <Binding ElementName="dataGrid"  />
                                    <Binding ElementName="dataPager" />
                                </MultiBinding>
                            </TextBlock.Text>
                        </TextBlock>
                    </Grid>
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

private void OnPageIndexChanged(object sender, PageIndexChangedEventArgs e)
{
    for (int i = 1; i < dataGrid.RowGenerator.Items.Count; i++)
    {
        var rowHeaderCell = ((dataGrid.RowGenerator.Items[i] as DataRow).VisibleColumns[0] as DataColumn).ColumnElement;
        (rowHeaderCell as GridRowHeaderCell).RowIndex = -1;
    }
}

public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
    var dataPager = values[2] as SfDataPager;

    if (dataPager.PageIndex == 0 || (int)values[0] == -1)
    {
        return values[0].ToString();
    }

    values[0] = (int)values[0] + (((dataPager.PageIndex + 1) - 1) * dataPager.PageSize);
    return values[0].ToString();
}

The index is displayed in RowHeaderCell based on the above customized style like below,

Take a moment to peruse the WPF DataGrid - Row Header documentation, to learn more about Row Header code examples.

View sample in GitHub