If you self publish you are no longer just the author, you’re the salesperson for your book. Do you have any experience selling? Did you love selling Girl Scout cookies? Do you like calling people and asking for money (as in fund raising?) Do you gladly spearhead the fundraising drive at your school, synagogue, church?
Be realistic. 20,000 units is a huge number of books. It’s a hard number to reach even if you’re published by a big publisher, with an accomplished sales force and established avenues to the retail market.
This post is not to dissuade you from self-publishing. Have at it with all your might. BUT be realistic about what self-publishing is, and what it can accomplish. And more important what it can NOT accomplish.