I have 2 credit cards with a zero balance. Will my score increase or decrease if I keep them open or close them?
Are those your only credit cards? Likely decrease if you close them, otherwise YMMV.
3-4 credit cards is generally considered to be the sweet spot in terms of maximizing your fico score. After that, most people see diminishing returns.
With that said, it might be advantageous to add another card, rather than closing your existing one(s).