There are 3,979 pledged delegates available during the Democrat primary process. Each state awards pledged delegates on a proportional basis at the statewide and district level. For a candidate to receive delegates, they must receive 15% of the vote at the respective state/district level.
For a candidate to win the Democratic nomination for President on the first ballot at the Democrat National Convention, they must receive a majority of delegates, 1991 delegates.
If no candidate receives a majority of the delegates on the first ballot, it would be considered a "brokered convention."
The 3979 pledged delegates would now have the ability to change their vote on the 2nd ballot, and 771 automatic delegates (superdelegates) would also be permitted to vote on the 2nd ballot. The nominee would be the person that receives a majority of all 4750 delegates. If a brokered convention takes place, it may take multiple votes/ballots at the convention to determine a nominee.