I have no custom threads settings in my slapd.conf. Currently slapd is running with 12 threads on my system with 8 cores (with hyperthreading).
I suggest reading the slapd.conf(5) man page and looking at the threads directive then. ;)
--Quanah -- Quanah Gibson-Mount Platform Architect Zimbra, Inc. -------------------- Zimbra :: the leader in open source messaging and collaboration