These notes might help...
Audio
The Mac can support stereo output through the built in sound system. If you want to support more than stereo output (front and rear stereo plus a sub for example) you need to add a multi channel audio output system. There is a lot of information about the audio interfaces which HW users have used on the forum. These devices connect to the computer over a USB connection or for some devices via a Thunderbolt connection. These audio interfaces support multiple independent output channels to which HW can send audio: the outputs from the audio interface connect to the amps and speakers. The number of outputs depends on the device (from 2, 4, 6, 8, to many...), the costs and quality vary widely. You need to decide how many output channels you want and how much to spend. You probably need at least 6 channels...
Monitors / touch screen
You need a USB port for each touch screen (for the touch input). With Macs people find they need third party touch screen drivers (Touch-base) for them to work reliably. The video for the screens each need an independent connection. The Mac’s video out may support two monitors or you may need to connect one of them via Thunderbolt or via USB.
Possible USB usage
USB 1 external disk
USB 2 Audio interface
USB 3 Powered hub a. Touch 1, b. Touch 2, c. Hw dongle d. Console midi
USB 4 Second monitor
Iain