Ideally you would want both quality and quantity. However, if push comes to shove, it is better to have fewer high-quality medical school letters of recommendation than lots of generic, lower quality letters.
For example, if the medical school you are applying to requires at least six letters of recommendation but allows up to ten, it is better to have six outstanding letters from passionate, committed writers who know you well, than to have ten mediocre letters from people who do not really know you and are only begrudgingly committing to creating them for you.