Do it. I'm doing 6 (bio, chem, electro, ICT - did the A2 this year, maths, and further maths). It was managable but due to basically not working throughout the year I got myself in a situation where I had a week to revise both bio and chem (and I knew next to nothing at this point) so I had to prioritise and learnt bio but ran outta time and had one day to revise chem so consequently aced bio and flunked chem.
Basically, do it. But work harder than I did.