I have a collection of business cards from my therapist, it's not huge just one of each type she has had over the years as she has changed locations or just the layout of the card. I could have a LOT of them if I wanted it is her traditional practice for her to give a client one of her cards that the next appointment written on the back. I never took any as a reminder for my next appointment, don't need them.

I also keep a voice mail that my therapist left on my phone to change an appointment.

And both the card collection and the voice mail are nice things to have