You need initially a very high density and a lot of mass to trigger the black hole formation. Once you are over this threshold, all the mass falls to the center and creates a "singularity" and the final density is "infinity".

Any additional mass that enter the event horizon falls to the center and the density is still infinity.

(The event horizon get bigger when you add mass. You can image the black hole as a big sphere that is almost empty, except for a few things that are quickly falling to the center. And you have at the center all the remainder mass gathered at one point.)