ffmpeg video grid

ffmpeg -y -i 1.mp4 -i 2.mp4  -i 3.mp4 -i 4.mp4 -i 5.mp4  -i 6.mp4 -filter_complex " nullsrc=size=1920x1080 [back];[0:v] setpts=PTS-STARTPTS,scale=960x540,crop=w=640[0];[1:v] setpts=PTS-STARTPTS,scale=960x540,crop=w=640[1];[2:v] setpts=PTS-STARTPTS,scale=960x540,crop=w=640[2];[3:v] setpts=PTS-STARTPTS,scale=960x540,crop=w=640[3];[4:v] setpts=PTS-STARTPTS,scale=960x540,crop=w=640[4];[5:v] setpts=PTS-STARTPTS,scale=960x540,crop=w=640[5];[back][0]overlay[a];[a][1] overlay=shortest=1:x=640:y=0[b];[b][2] overlay=shortest=1:x=1280:y=0[c];[c][3] overlay=shortest=1:x=00:y=540[d];[d][4] overlay=shortest=1:x=640:y=540[e];[e][5] overlay=shortest=1:x=1280:y=540[out];[0:a]volume=0.7[0a];[1:a]volume=0.7[1a];[2:a]volume=0.7[2a];[3:a]volume=0.7[3a];[4:a]volume=0.7[4a];[5:a]volume=0.7[5a];[0a][1a][2a][3a][4a][5a]amix=inputs=6[aout]" -map "[out]" -c:v libx264 -preset ultrafast -crf 20 -map "[aout]" -c:a aac out.mp4