I was told a long time ago that having too many cards can negatively affect your rating, especially if the total amount of credit available was more than your income.
IIRC the best credit builders are one or two cards paid off regularly, and a couple of utility type bills/mobile phone contract that's paid promptly as it shows both where you live is constant, and that you're managing your income vs outgoings.
Personally I have two, but use them as delayed debit cards, everything I buy online* or where I can't walk out of the store with it is on them (as are large purchases), but it's always paid back in full at the end of the month.
*I've had my card details taken via hacked websites/payment processors about 8 times (not happened for a while**), so I'm extremely leery of using my debit card online at all, as at least when it happens to the CC it's the banks money and problem, not my current account getting emptied.
**It's now going to happen again, probably next week as I've just cursed myself saying that
![Stick Out Tongue :p :p](/styles/default/xenforo/vbSmilies/Normal/tongue.gif)