the.com/critical mass

means The minimum amount of something — people, momentum, material — needed for a process to become self-sustaining and keep going on its own.

from Born in nuclear physics: the smallest amount of fissile material (like uranium) needed to sustain a chain reaction once it starts. Below critical mass, the reaction fizzles; at it or above, it keeps going — or runs away entirely. The term moved into everyday language around the mid-20th century, once the atomic age made it common vocabulary, and people started applying it to anything that needs a tipping point of participants or momentum before it 'goes critical' on its own.