alt Hacker NewsDisplayLink creates virtual displays on CPU and stream compressed feed over USB. That takes non-negligible amount of CPU time and RAM bandwidth, and it makes sense to give it a max concurrent stream count(set aside which of 4 or 6 are more reasonable).
While true, modern implementations use hardware blocks for acceleration AND for just terminals, with spare displays, you'll likely be using lower resolutions at lower refresh rates.
We've used a series of n150 based mini PCs (priced at $200 each) to power three dashboard displays at the same time. I guess it's a matter of calculating power usage and figuring out how that translates over 4 years