It even surprises me to see how many candidates have thrown their hats into the Presidential ring for 2020. Elizabeth Warren, Tulsi Gabbard, Julian Castro, and Kirsten Gillibrand. The Guardian lists three others of whom I have never heard: State Sen. Richard Ojeda (D-WV), Businessman Andrew Yang, and Former Congressman John Delaney. They also list at least 20 other candidates including Joe Biden and Bernie Sanders.
One reason why so many are jumping in early is that they are salivating at getting a chance to take on the unpopular Trump. Another is that the lesser known candidates have to start early raising money and organizing to be competitive. Sanders' 2016 campaign has provided a model for how to raise large amounts of money without relying on corporate and wealthy donors.
I'm reminded of the 2016 campaign when there were 16 Republican candidates (including one woman, one African American, one Hispanic, and one Indian American as of August 2015 vying for the chance to take on Hillary Clinton (who might give it another go this time). They had to split the debates into a main debate and a "kid's table" debate based on poll numbers. By the time the Iowa caucuses rolled around the field had whittled down to 11. It's hard to imagine there being such a large field by the time Iowa 2020 rolls around but it likely will be the more diverse than the Republican field.