If you're considering Medicine, 6 A levels would probably be a bad option. Not only would you need straight As (to get into the top top unis), you need a ****load of work experience, volunteering and all that. They will eat up time. I'm doing 4 (and I did pretty well in GCSEs), and I literally find that I have no time in the weekdays because of all of the extra-curricular, work experience and volunteering I have to do.
I would say 5 maximum for Medicine. I'm not sure about History, but I imagine that 6 would still be too much.
Having said that, I do know people who got 6A*, but they're few and far between. I'd say go for 5 at first. if you're finding it easy, maybe try 6. If you're finding it hard, drop to 4. Who knows? Maybe you can get 6A* and fit everything around it? Unfortunately, you'll probably end up with zero social life and fun.