you can override the default behavior by implementing a custom method. Here's an example of how you can achieve this:

from odoo import models, fields, api

class StockPicking(models.Model):
    _inherit = 'stock.picking'

    @api.model
    def create_backorder(self):
        """Override create_backorder method to customize backorder numbering"""
        res = super(StockPicking, self).create_backorder()

        # Get the original delivery order
        original_order = self.env['stock.picking'].browse(res['res_id'])
 
        # Get the original delivery order number
        original_order_number = original_order.name

        # Split the number to extract the prefix and suffix
        prefix, suffix = original_order_number.rsplit('-', 1) if '-' in original_order_number else (original_order_number, '')
 
        # Increment the suffix by 1
        new_suffix = int(suffix) + 1
 
        # Create the new backorder number
        backorder_number = f'{prefix}-{new_suffix}' if suffix else f'{original_order_number}-{new_suffix}'
 
        # Update the backorder with the new number
        backorder = self.env['stock.picking'].browse(res['context']['active_id'])
        backorder.name = backorder_number

        return res

In this code, we override the create_backorder method in the stock.picking model. We retrieve the original delivery order number and split it into the prefix and suffix parts. We increment the suffix by 1 and create the new backorder number by combining the prefix and the incremented suffix. Finally, we update the backorder record with the new number.