The limit is actually PCI address space. There are issues with some system and OS combinations making the practical limit less than the total number of PCI slots available.
The last time I did this type of test (about 2 months ago) I hooked up 6 displays. 4 LCDs and 2 CRTs. Using a 9600 and two 9200 PCI cards in some random G5.
In the future, perhaps we'll have cards with more than two outputs and then you can really go nuts.
We've already announced some FireMV cards on the Windows side for people who are in to multi-display trading systems and such (or other 2D-focused uses). 4 outputs from one card.
Bruno