Plenty of people do 5 AS Levels and do very well. I myself got 5As. Remember that you can always take 5 and then drop one if you find the workload is too much after a few weeks.
However, you should be careful that you don't over-stretch yourself. 4As is probably better than 3As and 2Bs, and is certainly better than 2As and 3Bs. Taking on an extra subject could affect your grades, although of course it doesn't for everyone. It's all about you individually. No-one on TSR can judge whether you're capable enough.