Sometimes things are only clear when you already understand the principles. What appears as the obvious way things should work can vary quite a bit among people, as is illustrated by today’s question.
“I have a MIDI sequencer that has 64 tracks, yet with any keyboard I plug it in to (including the one it’s built in to) I am only able to get 16 parts to play. What’s up?”
Tracks and “parts” do not necessarily have a one-to-one relationship. Each different MIDI part requires a separate MIDI channel. The MIDI specification allows for a maximum of 16 different MIDI channels to be sent down one MIDI cable. In order for a keyboard to be able to respond to more than 16 parts at once it would have to have more than one MIDI input port. There are some instruments available with this capability, but most are limited to doing 16 parts.
Sequencer tracks, on the other hand, can theoretically be infinite. You may have 15 different tracks all doing different elements of a drum part, for example. All of those drum sounds can generally be on one MIDI channel, which means you still have 15 channels left. MIDI channels and keyboard “parts” (different sounds that can happen at once) do not necessarily have a one-to-one relationship either. Sometimes a keyboard may have only eight or fewer parts even though it can technically respond to all 16 MIDI channels, but generally these days you will find 16 parts or more available in most instruments.
One way you may be able to get more than 16 parts out of your system is to use the 16 parts you can set up in the keyboard with the built in sequencer, and also send a MIDI signal out to another keyboard with 16 more parts (your sequencer/keyboard may or may not be able to do this). In computer based sequencing systems it is common to use a multi-port MIDI interface to provide more than 16 parts. Each cable can have 16 parts so if you have an interface that can send on 8 discrete cables, for example, you can have 128 separate parts so long as you have 8 multitimbral devices to hook up to those cables.