Which audio cards (pci or usb) are supported is really down to the underlying OS (usb drivers & alsa version) so 0810 or 10.04 for LinuxMCE and in our case currently 10.10 for Dianemo. I don't know of a supported sound card wiki page as such (although a few a listed on the Squeezeslave page).
As to how many you 'put' in a hybrid well that comes down to how they are physically added ie slots or usb and also on the overall ability of the host system to cope with the load the multiple 'cards' willl place on system resources. Obviously you will hit power & throughput limits on USB connected 'cards' but we have so far not hit those limits at all as most motherboards/systems have multiple usb busses etc. You can mix PCI & usb sound cards in any way you like without limitation... other than the ones just mentioned that is.
All the best
I use 0810 for now ,and i'm not planning to move do 1004 yet because 0810 is working just fine.I was looking for usb audio on the wiki and i got this article
which has just to options and for the first card (cmedia usb) the author didnt manage to get it work like squeezeslave at all.
The cards could be used on PCI ,PCI-X x1 and USB so what i will find that i will use .On the Core i believe i will use one PCI or PCI-x and one USB (total 3 zones) and on the MD (NT330i) i will use USB (for total of 2 zones) with which i will have 5 zones (current zone status of the house).This will wait for some time coz i contacted the suppliers for Audio Cards and they said they will have cards next week (i dont believe that they will have in so short time but it is ok).
The core system resources i quite well , the MD has lack of resources but i believe that it could manage one USB audio card