We recommend carrying 3 cards or a similar sized load, such as:
ID + 2 debit cards + business card
2 cards + 2 bills (folded twice)
6 business cards
ID + 1 card + subway card + folded bill
You get the idea.
Carrying more than the recommended load is not advised. Too much can stretch the leather – okay if you always carry more, but a loose fit means you risk losing cards when carrying less.
If you're always going to carry more, start with only 3 cards, and gradually add one more at a time. Either this, or you may consider stretching the leather: Learn how in this post.