Why concat in ffmpeg 4.2.3 says too many arguments?

Question

I am trying to flip 180 degrees parts of a video file from action camera. When i was using

"[clpABCD][clpEF][clpG]concat[out1]" -" instead of "[clpABCD][clpEFG]concat[out1]" -"

ffmpeg said Too many inputs specified for the "concat" filter !!!! Why?

ffmpeg -i video_output_02.avi -filter_complex \
  "[0:v]trim=start=0:duration=516[clpA];\
   [0:v]trim=start=516:duration=1,setpts=PTS-STARTPTS[clpB];\
   [clpB]vflip, hflip[clpBf];\
   [clpA][clpBf]concat[clpAB];\
   [0:v]trim=start=517:duration=1204,setpts=PTS-STARTPTS[clpC];\
   [0:v]trim=start=1721:duration=155,setpts=PTS-STARTPTS[clpD];\
   [clpD]vflip, hflip[clpDf];\
   [clpC][clpDf]concat[clpCD];\
   [clpAB][clpCD]concat[clpABCD];\
   [0:v]trim=start=1876:duration=581,setpts=PTS-STARTPTS[clpE];\
   [0:v]trim=start=2457:duration=24,setpts=PTS-STARTPTS[clpF];\
   [clpF]vflip, hflip[clpFf];\
   [clpE][clpFf]concat[clpEF];\
   [0:v]trim=start=2481:duration=253,setpts=PTS-STARTPTS[clpG];\
   [clpABCD][clpEF][clpG]concat[out1]" -map [out1]  -crf 14  -vcodec libx264  -vb 20M  Hflip_2.mp4
ffmpeg version 4.2.3 Copyright (c) 2000-2020 the FFmpeg developers
  built with gcc 7 (Ubuntu 7.5.0-3ubuntu1~18.04)
  configuration: --enable-gpl --enable-libx264 --enable-libvpx --enable-libmp3lame --enable-libopus
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
Input #0, avi, from 'video_output_02.avi':
  Metadata:
    encoder         : Lavf58.29.100
  Duration: 00:45:34.41, start: 0.000000, bitrate: 22653 kb/s
    Stream #0:0: Video: h264 (High) (H264 / 0x34363248), yuvj420p(pc, progressive), 1280x720 [SAR 1521:1520 DAR 169:95], 22649 kb/s, 32 fps, 32 tbr, 32 tbn, 64 tbc
[AVFilterGraph @ 0x564f261d9340] Too many inputs specified for the "concat" filter.



ffmpeg -i video_output_02.avi -filter_complex \
 "[0:v]trim=start=0:duration=516[clpA];\
  [0:v]trim=start=516:duration=1,setpts=PTS-STARTPTS[clpB];\
  [clpB]vflip, hflip[clpBf];\
  [clpA][clpBf]concat[clpAB];\
  [0:v]trim=start=517:duration=1204,setpts=PTS-STARTPTS[clpC];\
  [0:v]trim=start=1721:duration=155,setpts=PTS-STARTPTS[clpD];\
  [clpD]vflip, hflip[clpDf];\
  [clpC][clpDf]concat[clpCD];\
  [clpAB][clpCD]concat[clpABCD];\
  [0:v]trim=start=1876:duration=581,setpts=PTS-STARTPTS[clpE];\
  [0:v]trim=start=2457:duration=24,setpts=PTS-STARTPTS[clpF];\
  [clpF]vflip, hflip[clpFf];\
  [clpE][clpFf]concat[clpEF];\
  [0:v]trim=start=2481:duration=253,setpts=PTS-STARTPTS[clpG];\
  [clpEF][clpG]concat[clpEFG];\
  [clpABCD][clpEFG]concat[out1]" -map [out1]  -crf 14  -vcodec libx264  -vb 20M  Hflip_2.mp4

The command that "is working"

ffmpeg -i video_output_02.avi -filter_complex \
 "[0:v]trim=start=0:duration=516[clpA];\
  [0:v]trim=start=516:duration=1,setpts=PTS-STARTPTS[clpB];\
  [clpB]vflip, hflip[clpBf];\
  [clpA][clpBf]concat[clpAB];\
  [0:v]trim=start=517:duration=1204,setpts=PTS-STARTPTS[clpC];\
  [0:v]trim=start=1721:duration=155,setpts=PTS-STARTPTS[clpD];\
  [clpD]vflip, hflip[clpDf];\
  [clpC][clpDf]concat[clpCD];\
  [clpAB][clpCD]concat[clpABCD];\
  [0:v]trim=start=1876:duration=581,setpts=PTS-STARTPTS[clpE];\
  [0:v]trim=start=2457:duration=24,setpts=PTS-STARTPTS[clpF];\
  [clpF]vflip, hflip[clpFf];\
  [clpE][clpFf]concat[clpEF];\
  [0:v]trim=start=2481:duration=253,setpts=PTS-STARTPTS[clpG];\
  [clpEF][clpG]concat[clpEFG];\
  [clpABCD][clpEFG]concat[out1]" -map [out1]  -crf 14  -vcodec libx264  -vb 20M  Hflip_2.mp4

Answer 1

The concat filter has an option to set no. of inputs. Its default value is 2.

Change to [clpABCD][clpEF][clpG]concat=n=3[out1].