Most of the objects visible in the deep field images are galaxies: The regions were selected for having only a few foreground objects from our own galaxy, the Milky Way.
The area imaged is about a 26-millionth of the entire sky. So, the total number of galaxies can be estimated to be the number of identified objects × 26 million.
Zoom into the darkest regions of the image using mouse scroll or zoom buttons, and it is likely that you will see objects that are not identified by the tool. This means that the actual number of objects is more than identified by the tool.
The Brightness Threshold is the pixel value above which the tool considers a pixel as bright, and below which it considers a pixel as dark.
The lower the Brightness Threshold, the dimmer the object which can be identified. However, if it is too low, some of the objects start getting clubbed into one object.
Try to select the Brightness Threshold so as to identify as many objects accurately as possible.