Hi, Julien Plaitin

from odoo import models, fields, api

class StockPicking(models.Model):

    _inherit = 'stock.picking'

    sale_order_number = fields.Char(string='Sale Order Number')

    @api.model

    def create(self, vals):

        """

        Overrides the create method of stock.picking model to customize the creation of new picking records.

        """

        if vals.get('origin'):

            sale_order_number = vals['origin']

            existing_delivery_orders_count = self.search_count([('origin', '=', sale_order_number)])

            vals['name'] = f"WH/OUT/{sale_order_number}/{existing_delivery_orders_count + 1:02d}"

        return super(StockPicking, self).create(vals)

Please refer to the attached screenshot.

I Hope this information proves helpful to you. 

Thanks & Regards,

Email:   odoo@aktivsoftware.com           

Skype: kalpeshmaheshwari