FYI, I tried in a new studio Nuendo6 with the latest Protools HDX converters, 3 cards for a total of 48 channels.
This topic come out on this forum a while ago with no exact answer about N6/HDX channels capabilities.
Avid told us that N6 was going to be limited to only the first two cards for a max of 32 in/out.
Well, N6 was able to see all 48 in/out and rout any input to any output.
To be honest I didn’t try to connect physically more than 32 channels at one time. But N6 was seeing and managing the third card just fine (33 to 48) while Avid told us the limit for any other DAW but PT was the first two cards.