Cronjob are executed in _run_job method (/odoo/addons/base/models/ir_cron.py)

def _run_job(cls, job):
   ...
   cron, progress = cron._add_progress(timed_out_counter=timed_out_counter)
   ...
   cron._callback(job['cron_name'], job['ir_actions_server_id'])
   ...
   _logger.info('Job %r (%s) processed %s records, %s records remaining',
                job['cron_name'], job['id'], progress.done, progress.remaining)

• an ir.cron.progress record is created inside _add_progress method, 
  
• the ir.cron.progress record id is set inside the cronjob context using key'ir_cron_progress_id'
• the previously created progress record is used to log the number of processed and remaining records

Inside our cronjob, we 

• get the progress_id related to this cronjob using the context key 'ir_cron_progress_id'
• browse the corresponding ir.cron.progress record, and
• update its done attribute, as per below
  

def periodic_cronjob(self):
    processed = 2 # stub the result of processed records
    progress_id = self.env.context.get('ir_cron_progress_id')
    progress = self.env['ir.cron.progress'].sudo().browse(progress_id)
    progress.write({'done': processed})

Then on the cronjob completion, the number of processed records will be correctly displayed.

odoo.addons.base.models.ir_cron: Job 'My job' (30) processed 2 records, 0 records remaining