From ballotpedia:
The 2020 Democratic presidential nominee will be selected by delegates to the Democratic National Convention, which will be held July 13-16, 2020, in Milwaukee, Wisconsin. The national nominating convention is the formal ceremony during which the party officially selects its nominee. The delegates are individuals chosen to represent their state or territory at the convention.
In 2020, there will be an estimated 4,532 delegates: 3,768 pledged delegates and 764 automatic delegates—more commonly known as superdelegates.
To win the Democratic nomination, a presidential candidate must receive support from a majority of the pledged delegates on the first ballot—an estimated 1,885 pledged delegates. If the convention is contested and goes to a second ballot or more, automatic delegates will be able to vote and a candidate must receive majority support from all delegates—an estimated 2,267 delegates. Roughly two-thirds of the delegates will have been allocated by the end of March 2020.
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Pledged delegates are elected during primaries, caucuses, or party conventions, and must express either a presidential candidate preference or an uncommitted preference as a condition of their election. Rule 13(J) of the Democratic National Committee defines a pledged delegate's responsibility:
Delegates elected to the national convention pledged to a presidential candidate shall in all good conscience reflect the sentiments of those who elected them.