Most applicants with average stats for their target range apply to 20 to 30 schools. Fewer if you have strong in-state public options or unusually high stats; more if your stats sit below the typical range for MD programs.
The composition of the list matters more than the raw count. Always include your in-state public schools, since they're generally your best statistical odds. Use the MSAR to check whether your GPA and MCAT actually fall within each school's recent matriculant range, and resist the urge to pack the list with 15 or 20 reach schools just because they're recognizable names. Building that list strategically is one of the highest-leverage things an advisor can do for you, since a mismatched list wastes both money and time regardless of how strong the rest of the application is.