For RDIMMs, a minimum of two chip select (CS) signals are required per RDIMM. This is necessary for RDIMM memory device programming. A single or dual rank RDIMM configuration will need two chip-selects while a quad rank RDIMM configuration will need four chip-selects.
For LRDIMMs, a minimum of two chip selects are needed for programming as well, but there is also a rank multiplication (RM) factor to consider for LRDIMMs. Rank multiplication is a programmable ratio between the number of physical ranks for an LRDIMM and the number of logical ranks for the controller. The number of logical ranks seen by the controller per slot is always 2 for UniPHY-based memory interfaces which is the same as Number of ranks per slot. The UniPHY-based memory controller supports the rank multiplication factor needed for LRDIMMs by increasing the row address bits. The Number of Chip-Selects per device/DIMM gets updated according to the number of extra row address bits.
An LRDIMM with 4 physical ranks will have a RM = 2, and the UniPHY-based memory interfaces
cs output will be logically driven by
An LRDIMM with 8 physical ranks will have an RM = 4, and the UniPHY-based memory interfaces
cs[3:2] output will be logically driven by
Diagrams can be found on the Altera Wiki: DDR3 RDIMM & LRDIMM UniPHY IP.