Bernie seems to have 25 -30% support according to the polls. If you add up what Biden, Buttigieg, Bloomberg, and Klobuchar have the total is more than what Bernie has.
Since B B B and K are considered moderates, if any of them drop out chances are their supporters will not go to Bernie.
Eventually the field will shrink to two or three and that is when we will have a better take on what kind of lead Sanders really has.