We think that the mass boundary between neutron stars and stellar mass black holes is around three solar masses.
The maximum mass of the neutron stars now is two solar masses and we may find a 2.6 solar mass neutron star in the future. That is based on the equation of state.
The stellar mass black holes may have different origins. Even the smallest stellar mass black hole originates in SN explosion, we do not know the details of the explosion.
Why do not we think a two solar mass BH, or even a smaller BH, could exist?
Why do we think that the companion of Taylor-Hulse binary is a neutron star instead of a BH?