So you basically want to create serial number for the line items right?

Edited:

This can be done by an onchange function on account move. assuming your field on account move line is 'serial_number ', 
class AccountMoveLine(models.Model):

 _inherit = 'account.move.line' 

serial_number = fields.Integer(string='Serial Number', compute='_compute_serial_number', store=True) 

@api.depends('move_id') 

def _compute_serial_number(self): 

​for line in self: 

​ ​if line.move_id: 

​ ​ ​lines = line.move_id.line_ids.sorted(key=lambda r: r.create_date)

​ ​ ​line.serial_number = lines.ids.index(line.id) + 1