A single USB controller can control up to 4 units (full Prisma with 4 boards). Detailed info of each board can be found with cgminer api.
Info in the OP says 8 devices = 32 units, same as with the Tube, because each hashing board has a 5 bit DIP switch = 32 different addresses.
As I tested, each prisma board would be identified as two boards (seems they purposely do this to make it compatible with ethernet controller). Also the 5th bit on switch is ignored by prisma board. So it's 16 prisma boards or 32 tube boards.