I did seven.
Are you allowed? Legally yes, you can do as many as you want, but sixth form/college will only provide up to 4. Which means you self study for 2 of your subjects which I assure you is not easy. A levels are way, way harder than any qualification you've done before. That's for sure. You will also pay for all the exam entries that are not the ones you're doing in Sixth form or college. That's between 15 and 60 pounds per exam per subject. My last bill racked up to £174.90
There is no point. I regret doing more than three. Unis don't care, unless you're applying to a point-based uni, in which case there'd be no point anyway because UCAS point based unis tend to have lower requirements anyway.
"But I'm predicted X at GCSE!"
Doesn't matter. I've seen people come out with 12 A*s and run down to C grades in A level. They are nothing alike. Completely different leagues.
So if you want to go for it, knock yourself out but you will struggle and you'll probably regret it as I did.