API Call counting

[rrvenki]

I'm using Face API in my app. Request a clarification. If the camera is pointed to a location to detect and identify faces, the code loops every 250 milliseconds to find if there are faces and starts identifying them.

Question is if the system is running 5 minutes and only 2 people come within camera view in that time, will there be 2 API call/transaction count or 5 x 60 x 4=1200 call/transaction count? (5 minutes * 60 seconds per minute * 4 calls per second).
To return "No faces detected", if there is a transaction count then it becomes a very costly solution. Not all commodity cameras in market can auto detect human face or movement. Or is there a way to locally determine to make a call to API from mobile/PC? In Raspberry I already built a proximity sensor to detect nearby movement after which I make a face detection. Issue is in mobile/PC. (@CarlosP-MS).