There are around ~3,000 cases per year in the United States[1]. Since cancer is the result of cell division gone wrong, it can develop pretty much anywhere in your body. However, areas where cell division occurs more frequently are more common.

[1] https://www.mdanderson.org/cancer-types/appendix-cancer.html

Apparently extremely rare, 1 to 2 per million per year in the US:

https://my.clevelandclinic.org/health/diseases/22118-appendi...

I had confused it with pancreatic cancer in my head, given the extremely fast timeline :(