I guess it depend’s how precious you are about sound quality, especially given that ceiling speakers are obviously already a compromise. It also depends on where you are ideally planning to place the units.
But as wizardofoz suggests you could have multiple low-cost Chi-Fi USB DAC / Amp units (that model chosen at random, others similar devices available) plugged into a single RPi running Ropieee.
The one advantage of going that route is that it would require less Rasberry Pi’s and might easier to-setup for a non technical user. Not that a RPi with an Amp HAT is hard to plug together. Some of those Chi-Fi USB DAC/Amps also have a RCA subwoofer output which might also be useful in some zones.
I guess it depends if you want to have all your speakers wired back to one (or two) locations. Or if you want to place the units near to each room / floor, in which case the individual RPi + Amp Hat route might make more sense. You’ll probably want to mix those RPi/Amp HATs (for ceiling / patio speakers) in with some regular RPi DAC or DiGi HATS, or just even use the USB output on the RPi/Ropieee into a regular USB DAC for you main audio zones.
One advantage to going with multiple/individual RPi’s is that you could use each individual RPi endpoint outside of Roon as a Spotify Connect or AirPlay endpoint when using Ropieee XL or HifiBerryOS.
Fairly sure that when using Roon with multiple USB outputs, on a single RPi each USB output will be a separate zone in Roon (I think @wizardofoz has already all but confirmed this). But when using Spotify Connect or AirPlay (via HiFiBerryOS or Ropieee) I think a single RPi will only appear as a single endpoint, even if multiple USB DAC/Amps are attached, rather than 4 x AirPlay zones or 4 x Spotify Connect zones. Maybe someone with the kind of setup could confirm what happens in this senario when using Spotify (librespot) or Airplay (shareport). But if that is important to you then you may be better of with multiple Rasberry Pi’s, either with an Amp HAT each or an external USB DAC/Amp each.
If you’re going with multiple RPi + AmpHats in one (or two) locations you should be able to power them all from a single (larger) PSU and save some costs there. I’d talk to HiFiBerry re. power requirements if thinking of going that route.
Lots of options, but largely depends on your specific requirements and level of technical ability re. what would work best for you.