Most of the CCTV Camera output format as DAV files. Using ffmpeg we can Convert DAV format to Mp4 format. The ffmpeg already a "Multiprocessing" one.
sudo apt-get install ffmpeg
winget install "FFmpeg (Essentials Build)"
Run the script in the Directory with all the DAV files, and create a Directory named "Converted".
python3 DAV2MP4Convert.py
The Script then exports the Converted files to the "Converted" Directory.