Maximum number of CPUs

configname: CONFIG_NR_CPUS

Linux Kernel Configuration
└─>Processor type and features
└─>Maximum number of CPUs
In linux kernel since version 2.6.12  
This allows you to specify the maximum number of CPUs which this
kernel will support. If CPUMASK_OFFSTACK is enabled, the maximum
supported value is 8192, otherwise the maximum value is 512. The
minimum value which makes sense is 2.

This is purely to save memory: each supported CPU adds about 8KB
to the kernel image.