That's up from the previous year's SEC filing when Google only named two: Yahoo! and Microsoft. The latest dopcument also makes a point of claiming competition is only one click away.
I figure that the increased emphasis on supposed competitors is because Google is under well justified scrutiny by antitrust regulators and it is trying to make the case that it faces competition on many fronts.
Picking up on my discovery, Clint Boulton, who follows Google for eWeek and also writes a blog called Google Watch, put together a slide show looking at each of the 10 companies Google now makes a point of claiming it competes with. Mike Swift at the San Jose Mercury News also has a good article about Google's new spin on its "competition."
Boulton's eWeek show is worth a look.