Each DCU can contain 16 or 32 GByte of memory. There are 64 DIMM slots in each CMU, or 32 DIMMs slots in each CMP. See Understanding DCUs.
A number of factors affect this decision:
The size of the DIMMs in each CMU.
The number of CMUs in each DCU. See Determine the Number of CMUs in Each DCU.
The LDom configuration on each DCU. See Determine the LDom Configuration for Each PDomain.
The configuration PDomain on each server. See Determine the PDomain Configuration on Each Compute Server.