We get asked this all the time. It’s almost the wrong question. There is no maximum per se, but I wouldn’t plan on trying to keep more than 15 cards in the Slimmy. At that point you might as well have a big old George Costanza wallet anyway because you loose the sleek nature of the Slimmy. You also wouldn’t want that much in your front pocket. American Express actually circulated a warning this summer saying that men carry too much in their wallets leaving them very exposed to theft and then identity theft.
I used to be an 18-card, 2-months-of-receipts, lots-of-extraneous-crap guy with a big butt brick. I would use the Slimmy when I was going out and just grab the essentials, but eventually I never went back. I still have the old wallet full of loyalty cards from burrito stands and what not, but I keep a pretty trim Slimmy that has everything I need everyday and is small enough that you can’t tell I have it unless you frisk me.
I keep 4 bank/credit cards, license, registration, health insurance card, car insurance card, 1 to 5 business cards, and as many as 10 bills and a days worth of receipts.
Slimmy reminds you to take stuff out of your wallet. It’s so appealingly sleek when it has only the necessities in it. Once you have a few extra receipts dangling out, you remember to file them away. It reinforces discipline.