How many AP classes is considered "enough" for good colleges?

As long as you show rigor and proficiency then it will be good for college. For example, taking all AP classes for one year and gets let's say mostly B's in these classes is not as good as someone who balances out their schedule with AP and regular courses but gets all A's for that year. It's not really about how many AP's you take, but how many you can handle. Colleges don't require a certain amount of quotas for a given school. As long as you meet the requirements and your transcript looks good, you will be accepted. 15 AP classes is enough to get into an ivy league school if you're marks were good for those classes and showing involvement and leadership in high school.