Most multi-GPU farms run like 4-6 GPUs off a single machine. Although some specialized "BTC" motherboards have whacky scalper prices it's not hard to find other motherboards with OK multi-GPU support. You do not need a 'gamer' high performance motherboard to mine effectively. The only thing that really matters is being able to run as many GPUs as it's supposed to without bluescreening.
The power requirements are also not as high as you would expect from the typical power consumption of 30 series cards because it's overall more efficient to mine on much lower overall power consumption than you would get from a gaming workload. Mining just puts stress on different parts of the GPU than gaming and requires less power than a full throttle gaming workload. So you are able to run many more 3080s or what have you on a smaller PSU than you would be able to if you were using those 3080s overclocked and with a high power limit on gaming workloads. You just do not get that much extra hashpower if you go past certain levels of power consumption: it's easier / more efficient to just add another card.
Source: my workstation has 10 GPUs in it. Two are plugged into the mobo. 8 are plugged in via two PCIe switches.
