text: Title: How is the number of anchors calculated?
  Body: ### Search before asking
  
  - [x] I have searched the YOLOv5 [issues](https://github.com/ultralytics/yolov5/issues) and [discussions](https://github.com/ultralytics/yolov5/discussions) and found no similar questions.
  
  
  ### Question
  
  As an input i have image (640x640x3) and on output of the model is next: 1x25200x7, where 1 is a batch size, 7 is the number of classes+5 for bbox and objectness. But what is 25200?
  Can someone please tell me a direct formula based on input size to calculate it?
  
  ### Additional
  
  Also i found from model architecture that 25200 = CxCxB, but idk B,C. And i found that it depends on input size, for 320x320x3 it smaller but i don't remember how much.