If running stock clocks and not expecting to achieve max boost from PBO / XFR2 then a B450 Tomahawk / B450 Pro Carbon (which are 4 layer PCBs) is the minimum I would be looking at. The original post in this thread is asking about 12 or 16 core Ryzen 3000 to 5GHz (hypothetical) with a custom loop (i.e. low airflow from a fan in typical configurations).
As you know power goes up with the square of voltage and linearly with clockspeed. A chip that consumes ~ 105W at 3.6GHz base clock and 1.2V or so is going to consume (5GHz/3.6GHz) * (1.4/1.2)^2 =~ 1.9 x power which is ~200W. This is independent of process typically. That's why Ryzen has the boost algorithm based on workloads. If someone is using Blender , for example, that uses AVX instructions which stress the CPU more. In Ryzen 3rd gen the AVX2
support is improved such that it is wider : it will likely get hotter but get more work done with half the clockspeed. The use case for 12-16 cores is typically rendering or parallel workloads that can actually utilize the cores, otherwise it's normally 6 or 8 cores mentioned.
Ryzen algorithms currently use PPT (package power tracking), TDC (thermal design current) , EDC (Electrical Design Current), and PTC (platform thermal control) as limiters. There's no indication of this changing.
= package power of the CPU , default =~ 142W for X470
= power supply of motherboard limits it , default = 95A
= electrical constraint of motherboard , default = 140A
= thermal solution of the CPU limits it (i.e. the CPU cooler) --- TJMAX= 85°C
If you look at hardwareunbox's testing the Tomahawk is fine at ~ 160W with a fan on it , otherwise if you intend to run it for over 5K hours (~ 4 years at 4 hours/day or ~2 years at 8 hours/day) it's probably not going to last since it would hit ~90°C even with R7 2700X at stock.