This is the number of rows in the data area of the sheet. For the number of rows in the column header, refer to ColumnHeader RowCount.
fpSpread1.ActiveSheet.Rows.Remove(0, 5); listBox1.Items.Add(fpSpread1.ActiveSheet.Rows.Count.ToString());
FpSpread1.ActiveSheet.Rows.Remove(0, 5) ListBox1.Items.Add(FpSpread1.ActiveSheet.Rows.Count.ToString())