vCenter updates are received with long polling. Long polling means that the application calls a vCenter for changes and hangs until a change is made or timeout occurs. This part is not configurable.
The second part is if a connector is being communicated via the vCenter protocol (for example if WEB, SSH, SMB are disabled, or as a fallback), then all operations are done with calls to the vCenter for file operations. This may generate a lot of calls, depending on number of endpoints connected with the vCenter protocol.
Thus, the "Maximum vCenter calls per minute (Leave empty for unlimited calls)" means would a limit for total amount of calls to a vCenter. Setting a very low limit may have a strong impact on operations, and usually setting any limit is not recommended.
If you are curious about vCenter call statistics, they can seen with the following suffix to your core server URL:/vmanage-server/#!/statistics