I did 5 at AS Level (6 if you include critical thinking, which was compulsory)... English Lang, English Lit, Biology, Art, German and Critical Thinking. I didn't find it particularly stressful. Art took up so much more time than the rest, so I ended up dropping it at A2, but I think if it had been a normal/ less time consuming subject I would have been able to keep it. I was needing to spend something like 10 hours a week on my Art so it was just too much, but other subjects you wouldn't spend anywhere near that amount.
It also depends on you as a person. I certainly didn't do 4 hours of private study per week per subject as someone else here has suggested (probably more like 1-2 hours if that, except for Art as I mentioned), and I still came out with AABB. All I did was the compulsory homework/ coursework that was set. But some people do need to do more private study than others. It does depend partly on your intelligence/ aptitude for your chosen subjects/ ability to learn and retain information.
Anyway there's no harm in taking 5 to begin with, you can always drop one if it's too tough.
Edit: Also, I don't know much about Law, but I would have thought if you want to study a Law degree you would definitely want to be doing English, and History as opposed to Geography.