Literary / formal, used to describe great crowds, especially in fixed collocations like 美女如云 (beauties as numerous as clouds), 高手如云 (experts as numerous as clouds), 观众如云. Carries an admiring or awed tone, not a neutral one.
memory
Picture a sky packed wall-to-wall with clouds, that visual density is the point. The same metaphor pattern appears in 挥汗如雨 ('sweat like rain'), 川流不息 ('flow like a river'). 如 = 'like', and many idioms use 如 + nature image for sheer scale.
