Why is why is there an upper limit to the mass of neutron stars?
There is an upper limit to the mass of neutron stars because if the mass exceeds a certain value, known as the Tolman–Oppenheimer–Volkoff limit, the gravitational force would overcome the pressure from neutron degeneracy and cause the star to collapse further into a black hole. This limit is estimated to be around 2-3 times the mass of the Sun.