I vaguely recall the number being encoded 120 different ways. In any event this is independent of the number of users, and proportional to the amount of new incoming content. It also isn't real time, and doesn't really matter how long it takes other than marketing deadlines when they announce a new season of something far in advance.

The recommendations don't show any evidence of being calculated frequently, nor do they have to be done in real time. There is certainly way less appearing to go on than a site like Reddit.

"There is a lot of processing going on in the background" doesn't really answer the question :-)