If you are using Linux, macOS or Windows pm2 is your friend. It's a process manager that handle clusters very well.
You install it:
npm install -g pm2
Start a cluster of, for example, 3 processes:
pm2 start app.js -i 3
And make pm2 starts them at boot:
pm2 startup
It has an API, an even a monitor interface:
Go to github and read the instructions. It's easy to use and very handy. Best thing ever since forever.