Use the concat demuxer.
First, create a text file “textfile.txt” with the filenames.
file '1.mp4' file '2.mp4' file '3.mp4' ... file '13.mp4' file '14.mp4' file '15.mp4'
Then, run the concat command.
ffmpeg -f concat -i textfile.txt -c copy -fflags +genpts merged.mp4
For this to work, all videos should have same properties such as codec, resolution, framerate, sample rate, etc.
If they are not, you can encode the concat.
ffmpeg -f concat -i textfile -fflags +genpts merged.mp4
Also see FFmpeg Wiki: Concatenate.