This biggest limitations that I have come across (other then slots)are:
- Available power per circuit breaker
- Generated Heat
- Air Flow
You can get into FPGAs, but then cost rises alot.
Don't forget the ATI driver limit of 8 gpu's... that is a show stopper.
So currently max 8 gpu's ... so a max of 4 dual gpu cards or 8 single gpu cards.
Most people who have pushed this can get 6 single cards in a 890FXA-GD70 using 5 16x->16x extenders and a single 1x->16x extender. The power supply requirement would be huge ... depending on the card ...
But for example 6 gpu's needs at minimum a very good quality 1200 watt PSU. Like a Seasonic 1250 or an Antec TPQ-1200 ...