Hi there
It's just a theoretical question since I don't have the money to do it ;-) However if you would built bigger mining rigs (let's say 30 - 60 cards), is it better to run small systems with 6-8 cards (Asus Prime z270, MSI Z170A, Gigabyte GA-H110 etc) or 13 gpu systems with a ASrock H110 Pro / Asus B250? You're a probably more flexible with smaller rigs regarding cards and psu but need more time to maintain the system. So what would be overall better would you say?
First, it's more common to refer to one "rig" as one CPU/mobo/RAM set/etc. and all associated cards hosted on that individual system. I'm going to interpret your 30-60 number as the overall "size" of your theoretical mining operation, regardless of how many rigs are used to host that number of graphics cards.
The "best" distribution is a matter of available hardware (not everyone can buy a 19-slot board and 6x P106 cards right now) and personal preference.
Running more GPUs on each rig may give you a decent advantage in price and operating costs. Keep in mind that riser costs almost always scale exactly with the number of cards you have, and PSU costs do to a similar extent (except for extremes, such as 1-2 cards per rig, since each rig always requires a minimum of 1 PSU), so those will not be the main areas for savings.
However, in costs for the motherboard, memory, CPU, and storage device, you can save. (The motherboard slightly less so, since motherboards that can support more GPUs are
generally more expensive.)
On the other hand, having more cards per rig can start to put too many eggs in one basket. If any part of the rig "base" fails, for example, a CPU, then you have more cards down until you can repair it (whether that requires buying a new replacement board, or just finding the time to swap out for an available spare, although if you have spares readily available for all rig base components, we'd have to ask why you don't make some extra rigs, since purchasing spares defeats the point of saving on the initial investment, and not using them only saves a negligible amount of power, compared to what a 30-60 card operation would require).
Metroid's post had a little truth to it. If one card throws an error, it can bring down the mining software/driver/OS/etc. This definitely isn't always the case, but it can happen. (The most common example is a dual-PSU rig, where the secondary PSU (which only runs some of the GPUs) fails and manages to down the entire rig.)
Setup headaches are also something to consider. With a good motherboard and a uniform set of GPUs, I generally haven't had scaling problems between 6-12 GPUs on Ubuntu. In both cases, setting up drivers has been an all/nothing thing, rather than an issue of getting all 12 GPUs recognized. However, others have reported much bigger headaches, and I can definitely imagine the 19-slot Asus board being a pain, especially since it still requires 6x P106 "mining-oriented" cards in order to make full use of the board's potential.
In general, as you add more GPUs per rig, things get harder at different rates. For example, going from 5 to 6 might not be as hard of a jump as getting from 13 to 14. (This also depends on the motherboards you use, setup will generally get more difficult once you try exceeding the number of PCIe slots by throwing in PCIe splitters and/or M.2 adapters, but YMMV all the same.)
How you weight/prioritize the above factors is a matter of personal preference.
