The Movement Politics Threshold

A new study finds that when 25 percent of people in a group adopt a new social norm, it creates a tipping point where the entire group follows suit. This shows the direct causal effect of the size of a committed minority on its capacity to create social change.

From here.

The paper is Damon Centola et al. "Experimental Evidence for Tipping Points in Social Convention." Science (2018) DOI: 10.1126/science.aas8827

Other studies have put the number at 10% to 40% and point to the depth of commitment and memory length as key factors.