You only get 15 of the business cards for the maid cafe (though there are only 10 maids to find), if you run out of the cards (as seems to be your case) you can go talk to Gemini at the cafe and pay 100 dollars to get more. A tip for giving the cards to the right people, play with the sound on, and listen for bell ringing/chiming when you encounter and NPC while on the train or exploring the city, mall, and park, you'll also find one of them in Shadowed Curios. If you hear the bell, then that means they're someone who will accept the card and go work for the maid cafe, there's about 2-3 for each of the first three locations I listed IIRC, unfortunately whether their appearance will be triggered or not is pure RNG, so it might take a while, especially the ones on the train, if you get really unlucky.