I'm doing 5, it's completely manageable. I think 6 is also doable, even with plenty of EC activities.
I don't really understand how people can't cope with <6 AS levels. Most people have around 50 hours (4pm>10pm weekdays + 2x10 hr weekends) of non-school awake time per week. My ECs shave that down to 34, but that's still 34 hours when I can work on 5 subjects. Ultimately, I think I spend ~20 hours a week doing nothing constructive at all (e.g. TV, TSR, Facebook, Computer games) and still I'm on top of my work. It's nothing about how smart you are, or how well you learn. It's all about how well you can manage your time and avoid procrastination. e.g. if you can't keep on top of GCSE homework and you're that person who hands in coursework on the very last day then maybe you should be doing 3 or 4 AS levels.
If you're struggling at AS then it's because you're reading this sentence instead of working. I won't believe that with 32 hours in school and 50 hours free time per week that anyone can't keep on top of 4 AS levels. If that's the case then you probably need to sit down and rethink your daily schedule.