It depends on your school/college, and the option blocks if you have any. In my school, the maximum you can take is 5, as there are only 5 option blocks.
If I were you I would stick to 3-4 AS-levels. If you take 4 AS-levels you will have an advantage on going to good universities, as they recommend (optional though) you to have 3 A-levels and 1 AS (drop the AS subject at year 13, and continue 3 A2's onto year 13). In addition, it reduces the workload at year 13, as you dropped one subject. If you take 3 AS-levels, it's better to continue onto year 13 with all of them, as most universities want 3 A-levels. 5 is a bit too many in my opinion.