I wasn't able to figure out how to preserve the existing caption, but I think it is just "Summary: x" or "Total: x" or something like that. The following code sample demonstrates how to use the ICustomSummaryCalculator interface to show the number of rows along with the summary value:

this.ultraGrid.DisplayLayout.Override.SummaryDisplayArea = SummaryDisplayAreas.BottomFixed;           
this.ultraGrid.DisplayLayout.Bands[0].Summaries.Add(
    SummaryType.Custom,
    new CustomSummaryCalculator(),
    this.ultraGrid.DisplayLayout.Bands[0].Columns["UnitsInStock"],
    SummaryPosition.UseSummaryPositionColumn,
    this.ultraGrid.DisplayLayout.Bands[0].Columns["UnitsInStock"] );

public class CustomSummaryCalculator : ICustomSummaryCalculator
{
    private int total = 0;

    void ICustomSummaryCalculator.AggregateCustomSummary(SummarySettings summarySettings, UltraGridRow row)
    {
        object value = row.Cells[summarySettings.SourceColumn].Value;
        if ( value is short )
            this.total += (short)value;
    }

    void ICustomSummaryCalculator.BeginCustomSummary(SummarySettings summarySettings, RowsCollection rows)
    {
        this.total = 0;
    }

    object ICustomSummaryCalculator.EndCustomSummary(SummarySettings summarySettings, RowsCollection rows)
    {
        summarySettings.Lines = 2;
        summarySettings.DisplayFormat = "{0}";
        return string.Format("Total: {0}{1}Rows: {2}", this.total, Environment.NewLine, rows.Count);
    }
}