you would need to add code for a counter within the repeat.
so for example, the default repeat would look like:
<?php do { ?>
<tr>
<td><?php echo $row_RecordsetName['columnName']; ?></td>
</tr>
<?php } while ($row_RecordsetName = mysql_fetch_assoc($RecordsetName)); ?>
the code with the counter to show the banner, would look like:
<?php $BannerCounter = 0; ?>
<?php do { ?>
<?php if($BannerCounter == 5) { ?>
<tr>
<td>Show banner</td>
</tr>
<?php } else { ?>
<tr>
<?php echo $row_RecordsetName['columnName']; ?>
</tr>
<?php } ?>
<?php $BannerCounter++; ?>
<?php } while ($row_RecordsetName = mysql_fetch_assoc($RecordsetName)); ?>