ffprobe - Frame - Count number of frames

ffprobe -v error -select_streams v:0 -count_packets -show_entries stream=nb_read_packets -of csv=p=0 input.mp4

NOTE:

-v error - This hides “info” output (version info, etc) which makes parsing easier (but makes it harder if you ask for help since it hides important info).
-count_frames - Counts the number of packets per stream and report it in the corresponding stream section.
-select_streams v:0 - Select only the first video stream.
-show_entries stream=nb_read_packets - Show only the entry for nb_read_frames.
-of csv=p=0 - sets the output formatting.
- In this case it hides the descriptions and only shows the value.

Get frame count from video file metadata

ffprobe -select_streams v:0 -show_entries stream=nb_frames -of default=noprint_wrappers=1 input.mp4

WARNING: Fast, but often returns N/A, not reliable.

ffprobe -count_frames -select_streams v:0 -show_entries stream=nb_read_frames -of default=nokey=1:noprint_wrappers=1 input.mp4

WARNING: Pretty slow, can take a minute for longer videos.

ffmpeg -i input.mp4 -map 0:v:0 -c copy -f null -

WARNING: Needs to fully decode video once, which is rather slow.