Predictably, there is no one best answer to the question of what is the best number of college applications, although the trend has been increasing in recent years largely thanks to the Common Application. Given its uniformity and techno-ease, the Common App makes it easy to apply to multiple colleges.
Among my recent clients I have had students apply to as few as two, and to as many as 15. According to NACAC, the National Association for College Admission Counseling, one in four students presently applies to seven or more institutions: http://bit.ly/rTzPpy
Why I recommend multiple applications
In my view there are several advantages to going with multiple applications: first, provided the schools represent a range of levels of selectivity relative to the applicant, it increases the odds of getting acceptances; secondly, it also allows for a solid comparison of offers of aid, permitting a school-to-school comparison of total costs versus all discounting.
Obviously, there is a cost involved in applying to several colleges, both in terms of time and money. The majority of schools charge an application fee averaging about $50. Students from limited income backgrounds can request a waiver of the fee if it would preclude their ability to apply. I feel there is a level where an extremely high number of applications can approach absurdity, but hopefully the time it takes to negotiate and monitor applications and parental willingness to pay will act to keep things in check.