Apply to those schools at which you would like to attend and which you feel that you may have a chance on attendance. As far as choosing which schools, look at factors such as average GPA/MCAT, in-state/out-of-state acceptances, research emphasis, location, cost, reputation, match lists, etc.
There is no disadvantage to applying to any school. Obviously, the fewer schools to which you apply, the fewer chances you have to be accepted. There is never a guaranteed acceptance at any school. Even 'low-tier' (whatever that means) schools can be difficult to get into. If the school accepts students from many states, is in a good location, has lower than average MCAT/GPA, is inexpensive, etc., it will sound ideal, and therefore they will get many more applicants. Schools like George Washington University can get almost 10,000 applications each year for the 150 students they take while University of South Dakotah might get less than 500 applications.
There is never an ideal number. The average number of applications per applicant for the 2001 entering class (2002 not yet available) was 11.6. About half the applicants were not accepted anywhere.
