Rheia is designed/sized using ManJS and the supply duct static pressures are calculated using an airflow network model (basically what Manual D tries to simplify, and Rheia methods are interchangeable in terms of physics/math with Manual D). The design software is a Rheia specific version of Right Suite Universal and allows for accurate system static pressure estimates. Like any engineering process, of course, there is the principle of garbage-in garbage-out, and residential HVAC designers are notorious for fudging inputs to get the answer they want rather than following/trusting the engineering process.
The math behind the scenes is no different than what the duct slide rule is telling you. It’s funny that engineers have slide rules on their desks like my grandpa did in the 50s/60s, but I digress. As you know, whether the flexible ducts are installed stretched tight will impact the system static pressure, which is why Rheia systems are commissioned and tested each time to ensure the installed system matches the design, which by the way, is an important aspect of building energy efficient and comfortable homes.
Rheia doesn’t recommend a specific number of ducts for a specified bucket of air because, well, it depends. The design process will flush out any issues related to, as you put it, “wall clutter”. For high-end clients with very large rooms, Rheia might not work because you end up with a lot of vents in that very large room. That said, word on the street is Rheia will be releasing double vents that combine at the register to reduce the wall clutter. It is a less efficient system materially and is really suited best to smaller production style homes and multi-family units. Not to say you can’t have multiples of Rheia systems in larger more complex homes, but that requires more planning and structural thinking. A great application of Rheia ducts is on Ducted mini-split units, because you can minimize the length of your duct runs and you also get zoning by default. If you are successfully integrating bulky conventional ductwork in your homes, you are skilled enough to do it and your clients can handle the cost of all of that, then keep doing what you’re doing!