In order for the CountThings from Photos app to give you accurate results, the photos have to be taken correctly. In every case, the photos have to be taken with the highest resolution available on your device and the objects from the image have to be focused.
Depending on what types of items you want to count and specifically what Counting Template you are using, the angle may be different. Here are some examples:
Tubes, Pipes or Bars
For the Tubes, Pipes and Bars the photos have to be taken directly in front of the stack