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Description of problem:
Build 389-ds-base-1.3.4.0-21.el7_2.x86_64:
# tail -n1 /etc/profile
export MAX_THREADS=100

nsslapd-threadnumber: 30
nsslapd-enable-nunc-stans: off

# ps huH p $(pidof ns-slapd) | wc -l
41

nsslapd-enable-nunc-stans: on
# ps huH p $(pidof ns-slapd) | wc -l
72

# ldapsearch -h localhost -D "cn=Directory Manager" -w Secret123 -b "cn=monitor" -s base  threads  -LLL
dn: cn=monitor
threads: 30

On the older version (389-ds-base-1.3.4.0-19.el7.x86_64):

nsslapd-enable-nunc-stans: off
# ps huH p $(pidof ns-slapd) | wc -l
41

nsslapd-enable-nunc-stans: on
# ps huH p $(pidof ns-slapd) | wc -l
142

# ldapsearch -h localhost -D "cn=Directory Manager" -w Secret123 -b "cn=monitor" -s base  threads  -LLL
dn: cn=monitor
threads: 30

Looks like nunc-stans created additional thread pool (30 + 100 + 12 service threads). And on a new version it's 30 + 30 + 12.

Nunc-stans creates its own event catching thread pool, and it uses the DS thread number config setting.  So you should see double the number of threads.

Closing this out as not a bug, feel free to reopen it if I missed something.