As such there is no hard limit in N99panel w.r.t. number of websites/vhosts you can have. Practically it is based on the resources such as RAM and CPU. RAM is the more critical factor and below we would explain the same.

The RAM usage can be divided into

• System RAM usage - About 500 MB can roughly be ascribed to it
  
• MySQL RAM usage - At minimalistic level, it will consume at-least about 200-300 MB
  
• PHP Processes - Each PHP process on an average will consume 100 MB RAM. It may consume more RAM if more libraries are compiled/integrated into the PHP process or based on programming logic, the RAM consumption can vary greatly. Also the PHP-FPM process lifecycle also has a role to play in it. If Varnish is enabled for a WordPress website under N99panel, it help reduce the number of PHP processes based on certain scenarios.
  
• Node.js processes (if any) - Typically node.js process are relatively light on RAM
  

So now if our system has 2 GB RAM, going by above estimates, should be able to handle about 12-13 concurrent PHP processes. Any processes more than that can lead to slow response or un-responsive system or OOM (out of memory) issues like killing of a process.

Similarly, a 4 GB RAM VPS, should be able to handle 26-28 concurrent PHP processes

A 8 GB RAM VPS, should be able to handle about 70-73 concurrent PHP processes

A 16 GB RAM VPS, should be able to handle about 140-150 concurrent PHP processes

Disclaimer : The figures and numbers mentioned above are just for simplifying a complex answer and for educational purpose only. The performance and other results can grossly vary in individual scenarios and work-loads. Therefore its impossible to predict these numbers.