It would definitely be wonderful if 20-25 gpus could work in a single system.
However, this is not easy at all. There are various limits of gpu numbers coming from bios, OS, drivers, power delivery...
Check this thread for reference:
https://bitcointalk.org/index.php?topic=64450.0was sticking to 5 gpus per system, on the cheap base systems I have (gd45 mobos, 8gb ram) but the idea of turning that into 20-25 gpus per base unit would be very entertaining! (Not that I could find enough GPUs to fill it at this time)
That does not count the hard coded to WinBlows 9 screen max. I mean 9 monitors so dual head cards can count as 2 screens. I'm not sure what ( if any ) Linux distros may have as a limit and the original eBay actually can only do 2 cards per that adapter. The white for one type and the black for PCIx16 cut down to the x1 speed ( read the fine print people!! ) so you would need 2x of them per slot to only doubble your GPUs and most likely 1 dummy plug on all but what you actually look at to get up to 8 ( best guess guestimate ) cards per average 4 pci/pcix16 mobo out there. Still... good luck actually finding that many that will play nice together.