You can select one, two, or four DCUs in each compute server. See Understanding DCUs.
A number of factors affect this decision:
The number of CMUS in each DCU. See Determine the Number of CMUs in Each DCU.
The amount of memory that is in each CMU. See Determine the Amount of Memory in Each DCU.
The configuration PDomain that you choose. See Determine the PDomain Configuration on Each Compute Server.