Hi,

You can use the following code to add sequence in the sale.order.line.

Python

from odoo import api, fields, models


class SaleOrderLine(models.Model):
    """
    Inherits the sale.order.line model to add a computed field 'sequence_number'
    that represents a sequential line number for each line in a sale order,
    excluding optional lines like display-only types.
    """
    _inherit = 'sale.order.line'

    sequence_number = fields.Integer(
        string='#', compute='_compute_sequence_number', help='Line Numbers')

    @api.depends('sequence', 'order_id')
    def _compute_sequence_number(self):
        """Computes a custom sequence number for each sale order line 
        in the order."""
        for order in self.mapped('order_id'):
            sequence_number = 1
            for lines in order.order_line:
                if lines.display_type:
                    lines.sequence_number = sequence_number
                    sequence_number += 0
                else:
                    lines.sequence_number = sequence_number
                    sequence_number += 1

XML

<?xml version="1.0" encoding="UTF-8" ?>
<odoo>
    <!--    Inherited sale order form view-->
    <record id="view_order_form" model="ir.ui.view">
        <field name="name">sale.order.view.form.inherit</field>
        <field name="model">sale.order</field>
        <field name="inherit_id" ref="sale.view_order_form"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='order_line']//tree//field[@name='product_id']"
                   position="before">
                <field name="sequence_number"/>
            </xpath>
        </field>
    </record>
</odoo>

Hope it helps