Frank.LeeDepends on the card type. If you're talking credit cards that have a raised surface for numbering it's definitely no more than 4 on each side. If you're talking flat surfaced cards like ID cards, Starbucks cards, metro cards etc, maybe more. 8 is a very safe limit to put.
I keep 4 credit cards on one side (alternated orientation) and it's tight. On the other side I have Clipper card, bank card, ID, starbucks card, and that has a tiny bit more room, but not enough for another card.
I keep my security card for work in the cash pouch because it's very thick.