How many textbook problems should I do?

Textbook problems, such as those in Atkins' Chemical Principles tend to be very limited and straightforward and going through large banks of them can lead you to practice steps as opposed to strategy. Many Olympiads utilize 'trick' questions that are similar to common textbook questions but can not be solved correctly from the practiced straightfoward methods.

Additionally, its often beneficial to practice a bit higher than your comfort level. This doesn't mean you should jump from simple textbook problems to IChO problems, but you shouldn't be afraid of trying past Olympiad exams. Start practicing as soon as you've absorbed the fundamentals.