Hi PCP, you can choose as many keywords as you wish, but this is often entirely self-defeating as we only deliver 10 ads on a page. Setting 10 keywords will not give you 10 ads for those keywords. In most cases, our advertising partner will only return ads for one of the keywords you enter, making the other keywords redundant anyway. It makes far more sense to carefully select one targeted keyword than go for a blanket approach when setting keywords.
I hope this helps.
Ed