Claude Monet’s Rouen Cathedral series contains about 30 paintings.
Monet painted the cathedral’s western façade repeatedly in the 1890s, concentrating on how changing daylight and weather altered its appearance. Rather than treating the building as a single fixed subject, he used it as a surface on which color and atmosphere could constantly change.
The series includes works made from roughly the same viewpoint but under different conditions. Monet often worked on several canvases at once, switching between them as the light shifted. He later refined some paintings in his studio, so the group reflects both immediate observation and subsequent adjustment.
The total is generally described as around 30 paintings, rather than a perfectly fixed count in every catalogue. This is different from asking which cathedral Monet painted: the subject was Rouen Cathedral in Normandy, while the notable fact here is the approximate size of the series.