Manipulating audio channels
https://trac.ffmpeg.org/wiki/AudioChannelManipulation

6 × mono → 5.1
Combine 6 mono inputs into one 5.1 (6 channel) output with the amerge audio filter:

ffmpeg -i front_left.wav -i front_right.wav -i front_center.wav -i lfe.wav -i back_left.wav -i back_right.wav \
-filter_complex "[0:a][1:a][2:a][3:a][4:a][5:a]amerge=inputs=6[aout]" -map "[aout]" output.wav

All inputs must have the same sample rate and format. If inputs do not have the same duration the output will stop with the shortest.