The most common credit system is the semester-hour system, where a typical full time course load is 15-16 credits per semester, and 120-128 credits to graduate. The most common credit value of typical courses appears to be 4, although 3 and 5 credit courses exist, and some colleges may have 3 or 5 credits as the most common credit value. Some colleges just count courses, with all courses being the same credit value. Such colleges may specify 30-32 courses to graduate if each course is equivalent to a 4 credit course at semester-hour colleges.
The typical pre-med courses include 11-13 BCPM* courses, or about 44-52 credits in many colleges. In addition, there are writing courses and social science courses that are required or recommended by medical schools, probably about 4-5 additional courses, or 16-20 additional credits in many colleges. As noted, a major in biology typically covers nearly all of the pre-med science courses (and the non-science courses could fulfill general education requirements), so it is “convenient” in that respect, although a biology major would have to take additional upper level biology courses to complete the major. But around half of applicants and matriculants to medical schools have primary undergraduate majors other than biology.
*BCPM = biology, chemistry, physics, math