Georgia O'Keeffe's work was shown at Alfred Stieglitz's New York gallery known as 291.
The gallery was located at 291 Fifth Avenue in Manhattan, and its address became its name. Stieglitz originally operated it as the Little Galleries of the Photo-Secession before the shorter name 291 became widely used.
291 became an important venue for modern art in the United States. Stieglitz exhibited avant-garde European artists there alongside American artists, introducing New York audiences to developments such as Cubism and abstraction. O'Keeffe's charcoal drawings were displayed there in 1916.
The gallery closed in 1917, but Stieglitz later operated other New York spaces, including the Intimate Gallery and An American Place. Those names are therefore related to O'Keeffe's career, but they are not the answer to this question. The number 291 specifically refers to the Fifth Avenue address and gallery.