Is there any ffmpeg command for knowing max bit rate of a video?
That max_bit_rate is a a tag written by some encoders, when set by the user in the export dialog box. It is not an indication of the maximum bitrate found if you were to survey the encoded bitstream.



It's possible to get a good idea of the maximum bitrate of a stream this way:

ffmpeg -i in.mp4 -map 0:v -c copy
       -f segment -segment_time 1 -break_non_keyframes 1 folder\seg%d.264


This will break up the video stream into 1 second segments. Sort the directory listing by size to find out the largest file. That's your maximum bitrate. Not totally accurate as it's not a rolling survey but should be close enough.