How many letters of recommendation are enough?

The general rule of thumb is that when a school requires two letters of recommendation (LOR), then who you decide to write the LOR should be determined by how long you have been out of your undergraduate college.

For example, if you just recently graduated (1-2 years out) then the Admissions Committee will probably expect that both the LOR's come from a professor at your undergrad.

If however, you graduated more than 2 years ago, then you should try to have 1 LOR from a professor and then 1 LOR from an employer.

These guidelines are general at best and when you have questions where your situation doesn't fall into the above examples, you should contact the Admissions Office for further counseling.