CONFIG_NR_CPUS_DEFAULT_64 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

CONFIG_NR_CPUS_DEFAULT_64 (not configurable)

configname: CONFIG_NR_CPUS_DEFAULT_64

Linux Kernel Configuration
└─>Kernel type
└─>NR_CPUS_DEFAULT_64
In linux kernel since version 2.6.21 (release Date: 2007-04-25)  
is depended by
CONFIG_SGI_IP27