Please see the attached program.
It fails with the following output:
spawning thread #0
spawned thread #0 (thr_id=8053400)
spawning thread #1
spawn failure: Resource temporarily unavailable
Exit 71

ktrace shows the following information:
 27360 thread   GIO   fd 2 wrote 19 bytes
       "spawning thread #1
       "
 27360 thread   RET   write 19/0x13
 27360 thread   CALL  kse_create(0x804bf0c,0)
 27360 thread   RET   kse_create -1 errno 67 Too many processes

It seems that the problem occurs because flag THR_NEW_LWP, included into default set of ACE thread flags,
is interpreted by ACE library as an instruction to call pthread_setconcurrency with concurrency level
set to current number of threads created through ACE. FreeBSD libpthread refuses to set
concurrency higher than a number of CPUs available. ACE library treats pthread_setconcurrency() failure
as a fatal failure for thread creation.

Any solution for this problem should probably be submitted to ACE maintainers for
inclusion into ACE platform-specific source base.

Fix: Workaround: use libthr as a threading library

Solution 1: change a meaning of THR_NEW_LWP on FreeBSD as term "LWP" is not directly
applicable to FreeBSD threading model. Reasonable approach is to alias THR_NEW_LWP
to THR_SCOPE_SYSTEM.

Solutuion 2: do not treat failure of pthread_setconcurrency() as a fatal error
for thread creation.

No patches, sorry.
How-To-Repeat: 1. Change the attached program so that thr_num variable is greater than the number of CPUs on your test machine.
2. Compile the attached program:
$ c++ thread.cpp -o thread -I /usr/local/include -L /usr/local/lib -lACE -pthread
3. Make sure that libpthread is used as your thread support library:
$ ldd thread
thread:
        libACE.so.5 => /usr/local/lib/libACE.so.5 (0x48077000)
        libstdc++.so.5 => /usr/lib/libstdc++.so.5 (0x48196000)
        libm.so.4 => /lib/libm.so.4 (0x4826c000)
        libpthread.so.2 => /usr/lib/libpthread.so.2 (0x48283000)
        libc.so.6 => /lib/libc.so.6 (0x482aa000)

4. run the program