In theory, the limit is the limit of your processing power. Ignoring infrastructure issues, pitch the SKR Pro, get yourself a nice, beefy linux box with a couple of multi-port ethernet cards in it, then hook those up to some Mesa Enternet breakout boards, and connect them up with standalone stepper/servo drivers. Drive the whole shebang with LinuxCNC and you’ve got an N-Axis, industrial grade CNC controller. It’ll even slave different motors together for you and everything.
But that’s not really what you’re asking, is it?
You can chain at least two, if not three motors together in series from one driver. They aren’t driven independently, but if you’re using them both for the same axis, it’s not that big of a deal, as long as you have a hard stop available to always start your machine from. That’s what V1 users were doing (and still do, for the most part) before dual endstops became possible. As far as chaining controllers, I don’t know how much, if any, work has been done along those lines. Maybe some custom software…
The printer you describe could (and probably is) driven with no more than 5 drivers. I expect the X and Y are squared via hard stops at start-up and the three Z motors are used for bed leveling. And it might be run with just three drivers, with all three Z motors in series (or parallel [ugh!]).