I have 4 right now, but only 1 science, so I need to get the 5th one (waiting for the response....the prof must be out of town).
As far as "good ones," I'd say 2 of them are "great." By "great" I mean not just letters that would say I'm a good candidate for med school, but also ones from people who know me REALLY well. I'm confident that all my letters would be positive, but I think only two of them will be "great." I never formed a very close relationship with my science profs (not a science major), so I wanted to have more choices so that I can send other letters in instead for schools that are not very anal about the 2 science letter requirement.