Hi, here's my case. I'm creating serial number for my parking inventory on basis of 'sector_id' which is Floors in my model. Here's how i'm creating automatically serial number whenever inventory is created.

my.py

@api.model
def create(self, vals):
    if vals.get('serial_number', _('New')) == _('New'):
        sector_id = self.env['sector'].search([('id', '=', vals['sector_id'])])

        record = self.env['ir.sequence'].search([('code', '=', 'park.inventory.%s' % (sector_id.code))])
        if record:
            vals['serial_number'] = self.env['ir.sequence'].next_by_code(record.code) or _('New')
        else:
            new_record = self.env['ir.sequence'].create({
                'name': '%s' % (sector_id.code),
                'code': 'plot.inventory.%s' % (sector_id.code),
                'prefix': "%s/" % (sector_id.code),
                'padding': 3,
                'company_id': False
            })

            vals['serial_number'] = self.env['ir.sequence'].next_by_code(new_record.code) or _('New')

    rec = super(ParkInventory, self).create(vals)

    return rec

Let me know if u need further help.