Hi,

can you try the below code for odoo 12

in . py

class OrdinalNumber(models.Model):

    _inherit = 'account.invoice.line'

    number_sequence = fields.Integer(
        compute='_compute_get_number',
        store=True,
    )

    @api.depends('sequence', 'invoice_id')
    def _compute_get_number(self):
        for invoice in self.mapped('invoice_id'):
            number_sequence = 1
            for line in invoice.invoice_line_ids:
                line.number_sequence = number_sequence
                number_sequence += 1

in .xml

<record model="ir.ui.view" id="view_invoice_number_sequence_form">
        <field name="name">account.invoice.inherit</field>
        <field name="model">account.invoice</field>
        <field name="inherit_id" ref="account.invoice_form"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='sequence']" position="after">
                <field name="number_sequence" string="No"/>
            </xpath>
        </field>
</record>