Best bet might be to look at the X79 mobos, the have more PCI lanes to better accomodate multiple GPUs
I got a Asrock Z87 Pro4, which wasn't too expensive, but for the PSU I got the biggest they had at 1475w and that's not going to be enough for more than 4 GPU's - I'm running R9 280x's and so far got three and am aiming at 4 in total. However, I can't seem to get them to run together without BSOD and error code 3b and I'm worried it's a MB limitation, though the error code points to the drivers - am currently downloading older drivers to test... That being said, so far the bets one to mine with has been the ASUS, but till I get them all working at the same time, I won't be testing fully and tweaking completely and hence don't know how it will be after I'm done . . .