The count includes all processes that are alive, that is started executing and did not die. It does not include any thread that was started by foreign code, unless it calls into Lisp, in which case Lisp automatically generates a matching Lisp process which is included in the count.
In general processes can start and die so the real count may change by the time the function has returned. The only guarantee is that the count was accurate at some point between the time
processes-count was called and the time it returns.
LispWorks User Guide and Reference Manual - 20 Sep 2017