~$ ffmpeg -i ~/Downloads/111.webm -vf fps=1 -f image2 ~/Downloads/img/%d.png
ffmpeg version n4.3.1 Copyright (c) 2000-2020 the FFmpeg developers
  built with gcc 10.2.0 (GCC)
  configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-amf --enable-avisynth --enable-cuda-llvm --enable-lto --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libdav1d --enable-libdrm --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libjack --enable-libmfx --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-librav1e --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-nvdec --enable-nvenc --enable-shared --enable-version3
  libavutil      56. 51.100 / 56. 51.100
  libavcodec     58. 91.100 / 58. 91.100
  libavformat    58. 45.100 / 58. 45.100
  libavdevice    58. 10.100 / 58. 10.100
  libavfilter     7. 85.100 /  7. 85.100
  libswscale      5.  7.100 /  5.  7.100
  libswresample   3.  7.100 /  3.  7.100
  libpostproc    55.  7.100 / 55.  7.100
Input #0, matroska,webm, from '/home/user/Downloads/111.webm':
  Metadata:
    ENCODER         : Lavf58.45.100
  Duration: 00:00:03.13, start: 0.000000, bitrate: 2083 kb/s
    Stream #0:0: Video: vp9 (Profile 0), yuv420p(tv, bt709), 1920x1080, SAR 1:1 DAR 16:9, 23.98 fps, 23.98 tbr, 1k tbn, 1k tbc (default)
    Metadata:
      DURATION        : 00:00:03.131000000
    Stream #0:1: Audio: vorbis, 48000 Hz, stereo, fltp (default)
    Metadata:
      DURATION        : 00:00:03.087000000
Stream mapping:
  Stream #0:0 -> #0:0 (vp9 (native) -> png (native))
Press [q] to stop, [?] for help
Output #0, image2, to '/home/user/Downloads/img/%d.png':
  Metadata:
    encoder         : Lavf58.45.100
    Stream #0:0: Video: png, rgb24, 1920x1080 [SAR 1:1 DAR 16:9], q=2-31, 200 kb/s, 1 fps, 1 tbn, 1 tbc (default)
    Metadata:
      DURATION        : 00:00:03.131000000
      encoder         : Lavc58.91.100 png
frame=    3 fps=0.0 q=-0.0 Lsize=N/A time=00:00:03.00 bitrate=N/A speed=5.04x
video:1650kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown


# В то же время число кадров:
~$ ffprobe -v error -count_frames -select_streams v:0 -show_entries stream=nb_read_frames -of default=nokey=1:noprint_wrappers=1 ~/Downloads/111.webm
75