As we know, the Schwarzschild radius and mass are proportional.
r_s = 2GM/c^2
If the density of a substance is constant, the mass and radius of an object made of that substance are proportional to the cube.
Therefore, if the radius of an object doubles, the Schwarzschild radius increases by a factor of 8.
In other words, can a system made of multiple neutron stars rotating around each other become a black hole?
Please let me know if I am wrong.

I thought that a black hole with a very large Schwarzschild radius could have a very low density.
If you replace M in the Schwarzschild radius formula with density, you get the Schwarzschild radius formula depending on the density.
r_s = sqrt(c^2/(8.37Gρ))
If you substitute the density of water here, the Schwarzschild radius becomes 711334662km. So, a black hole with a radius of about 10,000 times that of the sun doesn’t have to have a very high density (the density of water).
If we extend this size to the entire universe, our universe could be a black hole and there would be no need for a singularity.
Please correct my foolish thinking.