Odoo-Build/snippets/mint_system.purchase.report_purchasequotation_document.sequence_in_table.xml

<data priority="50">
    <xpath expr="//table[1]/thead/tr/th[1]" position="before">
        <th>
            <span>Pos</span>
        </th>
    </xpath>
    <xpath expr="//tr[@t-foreach='lines']" position="before">
        <t t-set="index" t-value="1"/>
    </xpath>
    <xpath expr="//table[1]/tbody/tr/td[1]" position="before">
        <td>
            <span t-esc="index"/>
            <t t-set="index" t-value="index+1"/>
        </td>
    </xpath>
</data>