/// <summary>
/// 自动增长序号列
/// </summary>
public class DataGridRowIndexColumn : DataGridTextColumn
{
    /// <summary>
    /// 可以指定开始序号
    /// </summary>
    public int StartIndex
    {
        get { return (int)GetValue(StartIndexProperty); }
        set { SetValue(StartIndexProperty, value); }
    }

    public static readonly DependencyProperty StartIndexProperty =
        DependencyProperty.Register("StartIndex", typeof(int), typeof(DataGridRowIndexColumn), new FrameworkPropertyMetadata(0, propertyChangedCallback: StartIndexChanged));

    private static void StartIndexChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var column = (DataGridRowIndexColumn)d;
        if (e.OldValue == e.NewValue) return;
        int v = (int)e.NewValue;
        if (column.DataGridOwner == null) return;
        IList list = column.DataGridOwner.Items;
        TextBlock ele = null;
        int i = 0;
        foreach (object item in list)
        {
            ele = column.GetCellContent(item) as TextBlock;
            ele.Text = (v + i).ToString();
            i++;
        }
    }

    protected override void RefreshCellContent(FrameworkElement element, string propertyName)
    {
        base.RefreshCellContent(element, propertyName);
    }

    protected override bool OnCoerceIsReadOnly(bool baseValue)
    {
        return true;
    }

    protected override FrameworkElement GenerateElement(DataGridCell cell, object dataItem)
    {
        TextBlock text = (TextBlock)base.GenerateElement(cell, dataItem);
        int index = (DataGridOwner.Items as IList).IndexOf(dataItem);
        text.Text = (index + StartIndex).ToString();
        //https://www.cnblogs.com/RedSky/p/18215093
        TypeUtil.InvokeMethod(this, typeof(DataGridColumn), "ApplyStyle",
            new Type[] { typeof(bool), typeof(bool), typeof(FrameworkElement) },
            new object[] { false, false, text });
        return textBox;
    }
}

<DataGrid ItemsSource="{Binding Mods}" AutoGenerateColumns="False">
    <DataGrid.Columns>
        <local:DataGridRowIndexColumn Binding="{Binding Sort}" StartIndex="20"/>
        <DataGridTextColumn Binding="{Binding Name}"/>
    </DataGrid.Columns>
</DataGrid>