man make-kpkg says:
If defined, this environment variable sets the concurrency
level of make used to compile the kernel and the modules set
using -j flags to the sub make in the build target of make-kpkg.
Should be a (small) integer, if used. You can get the current
number of CPUs using the command:
grep -c '^processor' /proc/cpuinfo
This grep command does not give a useful answer on machines that use the
"hotplug" cpufreq governor, which switches off unused CPUs; these switched
off CPUs do not appear in /proc/cpuinfo. A better way to get the
CPUs is the nproc command. In this case, "nproc --all" would be appropriate.
(nproc is in coreutils, so everyone should have it.)