In your Outlook account, please right-click on the mail folder 'Deleted Items'. An option list will be displayed, in which you should select 'Empty Folder'.

After you click on the 'Empty Folder' option, you will get a confirmation pop-up. Do not empty/delete the mails in the trash folder. We just want you to note down the number displayed in the pop-up box. This is the count of emails present in your Outlook trash folder. 

Note: If the count of emails exceeds 10K, the exact count is not displayed. Just '10,000+' is displayed. Also, this option is available only in the new Outlook user interface.