I do three, and one is a BTEC, and the workload is hard enough for me. In my first year, I did five, but, along with getting 'average' grades, I had no social life/life outside of college. I wouldn't recommend it on any accounts, and with regard to getting recognition from universities, they are much more likely to recognise 3/4 outstanding A level grades then 6 average grades. You have to put in a lot of work as it is, even universities like Oxford only ask for 3 A levels. Give yourself at least a little time for 'you', and dont cause unessesary stess, unless it's vital you do them all.