According to this site
, the maximum amount of power that can be drawn from a 24-pin connector is 355 watts. All one would have to do is add up what a typical draw is prior to adding GPUs, and you could understand how quickly a hashing system + 4 GPUs would exceed that limit.
As for the motherboard question about number of slots vs. why the manufacturer doesn't list information. I think that is simply because none of them assume you will use 2+ cards. The reality is that the physical board cannot handle more than 2 or 3 cards without risers. Therefore, miners are utilizing these motherboards in ways that the manufacturer did not intend them to be used. Therefore, documentation doesn't exist outside of posts like mine