Limits the maximum number of processes which can be active as the current user before a fork request is permitted.
The count is not the number of children that this process may spawn. It is the total limit/number of processes running as the current user, which is checked before this process (or any descendants) may fork.
As such, it's safe to set this to 0 as long as no forks occur. This limit is not checked on execve, as no new PIDs are used.
In this example, you can see that things which do not result in a fork work just fine. This includes shell builtins (globbing, echo, read) as well as execing a new process. However, forking (as is done when executing a binary like head) does not work.
$ ./rlimit_nproc 0 sh
$ echo *
Makefile README.md rlimit_nproc rlimit_nproc.c rlimit_nproc.o
$ read line < rlimit_nproc.c
$ echo $line
#define _GNU_SOURCE
$ head -n1 rlimit_nproc.c
sh: fork: retry: No child processes
$ exec head -n1 rlimit_nproc.c
#define _GNU_SOURCE