This graph shows the total number of available schedulers and the number of online schedulers. Erlang’s schedulers schedule CPU time between your processes. The number of schedulers defaults to the number of CPU cores on the machine.
If you want to know more about schedulers, here’s a good article on Hamidreza Soleimani’s Blog on why the details of schedulers are important.
The number of processes and the limit are plotted here. The limit is the maximum number of simultaneously existing processes at the local node. If you reach the limit, the process will raise an exception. But the default Erlang limit is 262144, which should be high enough for almost all applications.