For a PHP based site, you may (on rare occasions) encounter an error with the number of FPM processes allocated to your site. The error message will look something like:
server reached max_children setting (5), consider raising it
This means that there are 5 concurrent processes running for your site. For a very busy site on a Virtual Private Server (VPS), this can be increased in order to allow increased traffic and visitors.
However, if your site receives less than a few thousand visitors a day, this can be a sign that you have an error with a plugin or extension or be a timeout in an external service you rely on. Before increasing the value, it's worth double checking your error logs to see why there are a high number of processes which aren't able to serve content.
Increasing this number increases the resources used for your site, which may require additional resources (such as memory) to be allocated to your server first. If you require this to be increased for your VPS,you can do so by following this guide.