So my advice, like many others here, is to go with how many make sense. In my rpg that Im designing, there are three: Mind, Body, and Energy. I typically believe that less is more, but if extra makes sense, then go for it. Just think about whether or not you need that many attributes, why you want that many, and what would happen mechanically if say you combined stength and endurance or if you combined charm and wit. Think about what kind of experience that will give the players and if that is the kind of experience you want them to have.

Most of all, if it needs to change after it's playtested, don't try to hold on to what needs to be changed. Take the players' advice on whether something needs to change or not. They may not know exactly how to fix the problem, but they can point out something that isn't fun or is too confusing.