25% Rule, Magic Third
Whether it's 10 or 10,000 people, changing the behavior, beliefs or norms of an entire group is hard, but new research suggests that in order to do so, we actually only need to convince 25% to start a persuasive domino effect. "Chance variation in a small number of early movers can have major effects in tipping large populations" — Noise
Related studies:
Centola, D., Becker, J., Brackbill, D., & Baronchelli, A. (2018). Experimental evidence for tipping points in social convention. Science, 360(6393), 1116-1119.
Referenced in:
Revenge of the Tipping Point by Malcolm Gladwell