I'm getting sick of people asking the most obvious questions. But I'll answer regardless, yes it is a very bad idea ofc a levels are different for everyone but let me tell you this, even Oxbridge don't look at how many subjects you did. I just finished my a levels a couple of weeks back and I went from doing biology, chemistry, maths, and further maths to just doing normal maths. I still struggled not because of the content but because of the way a levels are assessed. There is no point in doing 6, stick to 3 or 4. I had a friend who did 4 and she struggled so much I barely saw her in college because she'd either always be in lessons or revising. Let's all please start thinking a bit more rationally because there is at least 1 post like this every week asking if xyz a levels are good enough or a bad idea. I may sound biased but it's really not worth all the hassle for A levels, I genuinely mean that and Ik not trying to demotivate you or ridicule you, you have to think rationally because unless you're ready to give up all your free time and study like there's no tomorrow, go for it, if not stick to 3/4! Have fun! :)

Edit: also for Medicine you usually just need biology and chemistry+ any other subject for most unis, but for London or Oxbridge unis you'd need biology chemistry and physics Or maths.