MAXTHREADS

MAXTHREADS is the maximum number of threads that a single process can have active concurrently. If an application needs to create more than the recommended maximum in SAMPLIB, it must minimize storage allocated below the 16 M line by specifying C run-time options. For information about BPX1STL (the set_thread_limit service), see z/OS UNIX System Services Programming: Assembler Callable Services Reference.

You can set a system-wide limit in BPXPRMxx and then set higher limits for individual users by using the RACF® ADDUSER or ALTUSER command to specify the THREADSMAX limit on a per user basis as follows:
ALTUSER userid OMVS(THREADSMAX(nnnn))
Parent topic: Defining system limits