And the largest number of video cards is 13 on the same BIOS IIRC (mining cards without video is something else).

But how are the newer bioses for CPUs with a larger number of lanes than 16 (e.g. ThreadRipper)?

The number of physical slots is not important since simple $15 splitters solve that (not counting the reduced MTBF due to more minimal quality items).
I guess that PCI switches (those with $80 switch chips, e.g. the one with 8 USB riser connections) does not increase the number of video cards to the bios.

It would be really interesting if GPUs could be flashed into a "miner card" without video out (from the bios perspective). Then there could be more options for building 10+ card setups.

With only two rigs my experience is limited but it seems like 12 card rigs is a sweetspot when you know how to setup a balanced rig.
Testing you way forward with parts and settings, a 2-4 card rig is much more appropriate.

I tried to use 10 (on Z270P-D3) before i noticed that that mobo did not allow more than 9 (but one GPU got "ROI" by RMA so that solved itself).