Hi Laura,
Can you provide details? Perhaps screenshots of the map frame (I'm assuming that this is what is meant by "card frame") and the properties of the dynamic text element (what field is being used).
It is possible that you are conducting a count on a field where one of the features that has a null value. It is also possible that you are running into a bug. Without actually seeing your data I would only be guessing at why you are not getting the result you expect.
Can you contact Esri Support? They are better equipped to handle troubleshooting steps and gather bug data. They can be contacted at https://support.esri.com/en/contact-tech-support.
Thanks,
Tom