7 ~ Guerrilla Girls

In the spring of 1985, seven* women launched the Guerrilla Girls in response to the Museum of Modern Art's exhibition "An International Survey of Recent Painting and Sculpture", whose roster of 165 artists included only 13 women. The group anonymously created posters and protested against the institutions they felt were treating women unfairly.

Since then, the Guerrilla Girls have become art world sensations with their posters that ask things like: "Do women have to be naked to get into the Met. Museum?" and "When racism and sexism are no longer fashionable, what will your art collection be worth?" They've organized protests and conducted surveys of the percentage of female and African-American artists shown in museums across the country and abroad.

Source: columbiaspectator.com & wikipedia
* the number 7 was only found in 1 source