The current memcached for 3.0 would use the current scheme to determine this:
if the environment variable MEMCACHED_NUM_CPUS is set, it contains the number of threads to use. If not it'll try to use 75% of the cores reported on the system, with a minimum 4. (this patch got accidentally merged as part of the big patch sequence for moving the build over to C89 and cmake, its just been delayed to back out due to more "important" stuff going on..)
AFTER these default values is set, it'll look for the -t parameter which would override the default values.
Please note that this parameter require a restart of memcached in order to take effect.