You should always have your business cards with you. But what should you do if you just received a business card and you either ran out of yours or do not have them with you?
Thank the other person for their card and tell them you will forward your contact information to them.
Then, within 48 hours, 24 hours is preferred, follow-up by sending them written correspondence which includes your business card.
If you do not follow-up with this person, they may think you are not interested in building a business relationship with them and may damage any future opportunities you may have with them.