The Group of Pictures (GOP) in video files can vary between different video encodings. Some use a fixed GOP, some use varying GOP length. It should always be available in the file header information, so start looking there. Look up the file format specification to see where exactly.

15 I think

I think 8 or 16. In low delay application, you cas set to lower, e.g. 4, but when delay is not important, you can I think use 8 or 16. However, I think it is also related to the frame rate. If frame rate is higher (e.g. 60) you can use higher GOP.

It depends. In old videos with low fps, like an MPEG2 video in 15fps, it could be 8/16, while modern video with high fps will have a larger GOP, like an MPEG4-AVC video might with GOP size > 30/60.