Hi Medmars,

     You have to create one record under "ir.sequence" model.

then use this code in your python file.

a = 1
@api.model
    def create(self, vals):
        global a
            if self.env['ir.sequence'].search([('name', '=', 'Record Name')]).number_next_actual == 9:
                self.env['ir.sequence'].search([('name', '=', 'Record Name')]).number_next_actual = 1
                a+=1
            t = self.env['ir.sequence'].next_by_code('Model name')
            inc =self.env['ir.sequence'].search([('name', '=', 'Record Name')]).number_next_actual
            vals['field name'] = t
            vals['rayon'] = "R"+str(a)
            vals['casier'] = "C"+str(inc)
            return super(Class name, self).create(vals)

you can change the value as per your requirement.

Hope this will help you...Thanks!