Ctrl (or Cmd)+A, Delete 
I suspect if (rhetorically) there are 21K unread emails you probably won’t read them.
One of my managers before I retired (not my current life-long domestic manager!) had over 254,000 job-related messages in her Inbox. Then again, Outlook / Exchange was used as a permanent filing system because we corporately never bothered to pursue anything more appropriate for that type of data. She had been burned by a buggy Outlook rule, so she decided to use no rules at all. Auto-sorting rules (combined with brutal delete practices) was the only method that allowed me to keep a mostly empty Inbox with no unread messages anywhere at all. I had several thousand in total, but I kept on top of it (albeit with effort).