Stack Exchange network consists of 175 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
It seems that field_info_instance doesn't return such info...
I believe the 'cardinality' is stored at the field (base) and not the field instance. You should be able to use field_info_field to retrieve that information.
Required, but never shown