AIX's is in the file /etc/security/limits. Say you wanted to work with the root user, and limit / set the number of threads per processor. The file is made up of stanzas:
root: fsize = -1 core = -1 cpu = -1 data = -1 rss = -1 stack = -1 nofiles = -1 threads = 1024
Threads per processor is now set to 1024 (-1 means unlimited). To verify this is working, log in as root and type in:
ulimit -a
You should see threads show up as 1024.