I would agree with the other posters who said 3-4 cards. It can be done with more, but more than that becomes harder to manage for most people.
I had always been under the assumption that it was to have 3-4 cards as well. I did this for the past 8 years and my credit has been good, even when I was starting out (between 700-760 when I started checking it 3-4yrs ago).
As I have begun to learn more about credit through this website as well as creditkarma.com my opinion changed. CreditKarma states the following as one of the average score criteria;
"Total accounts is another measure of your creditworthiness. Consumers with more credit accounts generally have better credit scores because it means more lenders are willing to grant credit. This metric represents the total number of accounts listed on your credit report. A breadth of different account types is indicative of good credit." (CreditKarma)
They then go on to show a line graph that shows those with 21 plus credit lines have an average score of 689. Though this sounds low, the average score only decreases as you have less credit lines down to 619 for those with 1-5 lines.
I have apped for CSP, Chase Freedom, Chase Marriott Premier Rewards, and Amex BCE in order to increase my accounts and to gain some extra rewards. I have pretty much said goodbye to my older 3 cards and will mainly be using these new ones due to rewards.
Maybe general knowledge is changing about the number of cards? Is CreditKarma off?