What is the mass constraint that separates the largest Brown Dwarfs from the smallest true main-sequence stars?

The deuterium-fusion limit, around 13 Jupiter masses.

Brown Dwarfs are classified as substellar objects, often nicknamed 'failed stars,' because they fail to meet the critical threshold necessary to initiate and sustain stable hydrogen fusion, which defines a true main-sequence star. This dividing line is determined by the required core temperature and pressure. Specifically, objects between roughly 13 and 80 times the mass of Jupiter occupy this category. While they may briefly ignite and burn deuterium—an isotope of hydrogen—they cannot achieve the sustained core conditions required for standard hydrogen fusion. Below this deuterium-fusion limit (about 13 Jupiter masses), the object is generally considered a giant planet rather than a Brown Dwarf.