The metadata information of NB Frames can be broken down into its two parts. NB stands for "number" and it further defines the second part. Thus, simply put, NB Frames is the number of frames present in the stream of either a video file such as Mp4 or MOV, or an audio file such as AIFF or WAV.
NB Frames usually gives the total number of frames over the whole stream. If the number is unknown or could not be determined, it is usually given the value of 0 (zero/null).