If it's a stage effect and you have already passed out business cards. Well, make sure that you haven't passed business cards to a group of people, and now you have a group who need business cards. Get the prepared business cards + 20 blank cards and pass the blanks to the 10 people who need cards ( have extra since some might have lost theirs ) . Now, all you have to do is walk by, collect the business cards on Top of the prepared cards and keep a break or a short corner. As you come up on stage, very casually cut the audience's cards onto the table as if these are the blank ones. Wait, there's a subtlety coming in !! You are now left holding your prepared cards ( in your left hand for explanation purposes ). Now for the SUBTLETY !! You move back to the table, your right hand takes the audience's cards off the table and holds them up in the air as you say:" does anyone else need a card to write on ? ". If someone does, which isn't true since everyone has a business card, but if you person does, you pass him a blank card ( I purposely forgot to mention that when you replace the audience's cards onto the table, you also put a couple of blanks of the bottom. How ? By cutting a couple of cards after the break ).
Now depending on what routine you're doing, if a person does need a business card, you might have dead time, so if someone does need a card, it looks like you still need to collect some other business cards. Now collect about 10 more from the audience and when you're on stage say:" we only need half of these" and get rid of those 10 extra business cards ( identified by a break )
Oh well, it looks like a heck of a way to do it in text. But imagine the actions and it really is a simple procedure.
Just one more thing: If you wish to distribute cards to everyone, just do that. Then take out the prepared business cards ( with extra blanks on the top, separated by a short card in between ) and tour around the audience as they are writing and say " if anyone needs blank paper here are some ". Then proceed as before.
Maybe that would suit you.
