How to save detected faces according to the bounding box ID?

I work with this example: Detect and Track Multiple Faces. However I have no idea to save all detected face in each frame based on the

tracker.BoxIds.

This is my code:

...
...
for j=1:size(tracker.Bboxes,1);   
       J=imcrop(displayFrame, tracker.Bboxes(j,:));
       imwrite(J,strcat(sprintf('frame%d(%d).jpg',frameNumber , j)), 'jpg');
end
...
...

The code I provide here does not save all the detected faces in one frame. My input video contain a group of people that pass through a camera. So each frame will contain different number of face. How can I crop all different faces that appear in a frame?