In 2020 there are 3979 pledged delegates and 771 super delegates. Super delegates do not come into play until the second vote (if any).
To win the Democratic nomination, a presidential candidate needs to receive the support of 1991 pledged delegates.
In the case of a contested convention, the candidate receiving 2375 votes will become the nominee.
These states below have open primaries. The first number is the number of pledged delegates, the second number is the number of super delegates,
By these states alone, it is not mathematically possible to get the necessary 1991 pledged delegates to win the nomination.
That leaves 453 delegates that need to be won from the closed primary states. The remaining closed primary states total 2441 pledged delegates. As 453 more pledged delegates are needed, this means that at least 19% of closed primary voters would need to vote for the independent candidate.
We can repeat this exercise for different scenarios. If an independent candidate gets 100% of all open primary states, that candidate would only need 19% of the pledged delegates in the other states, or if an independent candidate gets 80% of the pledged delegates of an open primary state, that same candidate would need 31% of the closed primary voters.
Given that super-delegates are less likely to vote for an independent candidate, if these threshold aren’t met, the chances for an independent candidate to win the nomination become extremely small.