You are correct - more than eight modules requires the use of a second CompactRIO controller and chassis. With some R series boards you can have more than eight CompactRIO modules by using multiple cRIO-9151 expansion chassis. For example, the PCI/PXI 7813R has four DIO connectors, allowing you to connect four expansion chassis.
Currently the maximum number of C series modules per cRIO chassis is 8. To expand the number of modules and I/O channels there are a couple of options, but they do require some extra programming.
One option may be to use a R series plug-in card for your FPGA (using a desktop or PXI computer as your host) and then using up to 4 cRIO expansion chassis (cRIO-9151) attached to the R series plug-in card. This option gives you up to 16 C Series modules attached to one R series card (PCI/PXI-7811/7813). Of course this solution would not longer be a pure cRIO system and would be larger and not meet some of the environmental specs of cRIO. Whether this is feasible depends on your specific application needs.
The other option is to use multiple cRIO controller and chassis. The different cRIO systems can communicate with one another using Ethernet or Serial to share and exchange data. For tight timing and synchronization between chassis you can use digital I/O modules in each chassis to share clock and trigger signals. Using one NI 9403 module in each chassis you can share up to 32 timing signals. Using the NI 9401 module in each chassis you get 8 clock and trigger lines but have higher possible clock rates between systems compared to the 9403. See the following article for more information on synchronizing multiple cRIO chassis.
Some of the engineers at National Instruments are interested in discussing your application. May we have permission to look up your information and contact you?
If it would be all right to look up your contact information, someone from National Instruments would be happy to discuss possible options with you.