I doubt the literal equivalent 臨界量 is likely to work here.
I'm speaking in the sense of a community activity, where a critical mass of people is required in order for it to survive. e.g. if a group of 10 people meet weekly to do something (e.g. playing poker, going hiking, etc.) it's likely to continue to happen, while if only 2-3 people are interested it's likely that the desire to continue will disappear and it becomes difficult to encourage new people to join.
Is there something similar I could use to refer to this concept?