You know, eventually — I mean, rarely or ever — you may need to use multiple MIDI Through ports on your Linux workstation. If you want to, let's say, compare two VST's and the only way to get them to receive MIDI data is per via MIDI Through, you may want to use two separate ports, because nobody likes cacophony, I guess.
The way to do it is specifying a parameter to the module snd_seq_dummy, which provides us the MIDI Through ports. # modinfo <module-name> is a good way (besides Google) to tell which parameters the module takes. For snd-seq-dummy, it says:
filename: /lib/modules/4.19.0-5-rt-amd64/kernel/sound/core/seq/snd-seq-dummy.ko alias: snd-seq-client-14 license: GPL description: ALSA sequencer MIDI-through client author: Takashi Iwai <tiwai@suse.de> depends: snd-seq [...] parm: ports:number of ports to be created (int) parm: duplex:create DUPLEX ports (bool)
I do not know what “duplex” means in this case. But I'm pretty sure “ports” is what we're looking for. Then, you just go into /etc/modprobe.d, create some .conf file there and enter the following line:
options snd-seq-dummy ports=<the number of ports you want>
Save the thing and rmmod snd-seq-dummy && modprobe snd-seq-dummy or simply restart the computer, if you're feeling a bit Windows user today.