1) Generally, you need at least 4-5 accounts on your credit reports to maximize your credit score. Loans, credit cards, mortgages, etc. all count towards this number. Thus, yes, there could be a significant advantage to having more than 1 credit card if you don't have many other accounts.
2) To maximize your credit score, you usually need one credit card with a $0 balance. This is NOT the same thing as paying off your CC in full each month. Because, when they run your credit report, the chance of you having a $0 balance that day is slim. Thus, you should have one CC that you rarely use so that you'll have one CC with a $0 balance.
3) For you own convenience, you should have one CC at home that you don't carry with you. Then, if your wallet/purse is stolen, you can still live your normal life when you are waiting for the replacement cards. Demon-Xanth, below, had a good point that is related. If one CC is giving you problems (scammers, technical difficulties, etc), you can instantly switch to the other card.
#2 and #3 can be the same CC.
