Do note its the A's(actually A*s) everybody is talking about, not the number of subjects(. So you must keep this in your mind while deciding.
Why you take O-levels? for A-levels. 8A*'s can get you in pretty much any school you want. Why you take A-levels? to get into a university of your liking. 4A*s can get you pretty much anywhere. After 4 A*s, a 5th or more A* doesn't contribute as much as other things. For example, SAT scores/Entrance test scores/MCAT, proof of activities that show you are really into your field, letters of recommendation, personal statement, interview etc.
Now I'm not saying getting more A's wouldn't help, because it would really. But there are things that would help your application more than just a greater number of A's(for example a bad interview can guarantee rejection, no matter what else you have on your application) . But if you are sure you are willing to put some extra effort, then go for it by all means. Its just that quite a lot of people plan, but are not ready to go through it.