Hi Chan,
Can check and mention the code once again.

<tr t-foreach="o.invoice_line_ids" t-as="invoice_line">
<td class = "table_border text_center">
<span t-esc="i"></span>
<t t-set="i" t-value="i+1" />
<t t-if="i % 10== 0">
<div style="page-break-before:always;"></div>
</t>
</td>
</tr>