AMD (ATI) Maximum monitors supported (Link)
AMD's site Eyefinity FAQ page details maximum monitors supported for each series they offer.
http://www.amd.com/us/products/technologies/amd-eyefinity-technology/how-to/Pages/faqs.aspx#maximum-monitors
Important to note individual card manufacturers determine implementation. Both in number of ports & maximum number of monitors supported. In other words a card can have 4 ports yet only support 3 monitors (see last paragraph).
Find the series from AMD's FAQ then always check implementation by looking up card manufacturers website before buying.
Currently the norm for display ports is to need an active adapter after you connect two monitors. This is to do with the implementation of each card. Active adapters provide additional hardware only implemented for 2 monitors (specific combinations of ports) on the card. This saves both money & space for card manufacturers.
Also consider the memory available for each GPU. Some newer games may use more than 1GB when eyefinity resolutions are implemented & therefore benefit from newer larger memory cards.
I have a 5850 triple monitor setup at present. The maximum supported by card even though it has 4 ports. 58xx series can support 6 monitors so limit is implementation. I also have a 5870 eyefinity 6 2GB card I'm looking forward to trying out in the future. Will crossfire the 5870 & 5850. This will slow the 5870 to clock of 5850 reducing efficiency by only a small percentage. Could also overclock the 5850 to 5870 spec but personally prefer to under-clock for reliability & longevity.