I'm new to Node.JS, already Googled for aswers but there is a lot of ways of doing this. Do you have some sort of math to know when I'll need to scale my Node.JS server? And what tools you use for monitoring server performance and error track?
As @ThiefMaster said, this depends on your system. You need to monitor the requests for your site, the CPU load, memory footprint etc and decide if and when to add mode servers.
If you want to know more about Node.js performance read this good article: http://blog.3rd-eden.com/post/5809079469/theoretical-node-js-real-time-performance
This recipe can be found in it's original form on Stack Over Flow.