It needs at least 3 on a PC. They say in this very video what they use each core for. And then state that the game would not run and look this good if it was not for multiple cores. The game designers have designed the game so that if it was on less cores they would need 2 completely different games. Becasue they cant take any of the stuff out because it's all relevant to the gameplay.

1 core for physics

1 core for dynamic lighting night and cycle.

1 core which prepares all the information for the GPU.

1 core to run the rest of the games operations.

Actually no. Two cores is enuff.

It was in an article on Yougamers in 2007:

"So what's this "secret" we've been mentioning about the demo? Well, okay, it's not really that much of a secret but remember how Remedy ran Alan Wake in IDF on a quad core machine? Well, they showed it to us on a dual core processor in a machine with around 2GB of RAM (perhaps more) and a single ATI Radeon X1900 series graphics card - all running perfectly well on a massive HD screen with anti-aliasing enabled."

http://www.yougamers.com/articles/4489_remedy_interview_-_part_3_the_power_behind_wake-page5/
http://www.yougamers.com/articles/4489_remedy_interview_-_part_3_the_power_behind_wake/

I'd guess that on a Dual core CPU each core will run two threads, in total corresponding to the four tasks you listed above, selnor.