Hi,
You can use on all displays the maximum resolution supported. In our case if it is 1920x1080p then you can get 6 without issues.
If you want to create an Eyefinity 6 by creating a SLS panel at 5760×2160 that is not a problem either, that is one of the best selling features of this card.
Now the maximum resolution on each DP port is 4096×2160 at 60Hz thanks for to the DisplayPort™ 1.2 specification so in theory you can arrange each display at this resolution. The main problem with this resolution is that it might not be appropriate for some applications that require heavy use of the GPU as the array of multiple displays at 4K it might create an issue on performance.
We have made some videos on our youtube channel for Eyefinity 6 cards which you can see;
Playing 6 simultaneous video streams at 1080p on youtube. This is using the HD 7870 Eyefinity 6.
Playing BF4 with the Radeon R9 290X with the MST Hub, creating an Eyefinity 6 setup.
Testing Windows 8 Beta with Eyefinity 6