We found this document:
http://www.ofitselfso.com/BeagleNotes/UsingDeviceTreesToConfigurePRUIOPins.php
And it says “The number of possible inputs and outputs per PRU is fixed and each has 16 possible inputs and 15 possible outputs. Those are just the theoretical numbers - many inputs and outputs are not routed out to the P8 or P9 headers on the Beaglebone Black and the actual number of usable inputs and outputs is quite a bit lower.”
So which BB is capable of most usable number of PRU channels? For our project we need 30 realtime inputs at 5MHz.
More questions will probably follow 
Thanks!
Dom