....I was paranoid, so I did 30. I'm aware this is an excessive number, but I really bought myself a lot of peace of mind with it, and the ability to drop multiple pages a week during a big fight scene, and not totally destroy my page buffer.
I'd say your ideal number is in the middle zone between 'I have enough pages to cover a small crisis like an illness or technical issues' and 'I have few enough pages that I can't afford to slack off too much'.