^ I think drivers limit the max amount of cards on CFX to be four, but Windows 7's apparent limit is five. Anyway, for example you have a motherboard with six slots.
If you put one card in the first slot, and one card in the third, there will be a gap between them, which will help with airflow. Also, six slots is only enough for three way CrossFireX or SLI most of the time, as you have to remember that most cards are dual-slot, and by taking up two slots, you will only be able to fit three graphic cards in.
you said that the gap between them is for cooling.i said they can put 4 solt as far as 6 solt so the gap between them is like 6 solt and it has same result for cooling.6solt is more expensive than 4 solt.so i think that cooling is not main reason for that