One Xvfb server should handle many connections well. One thing you want to do is start the server with the -noreset option. Without it, a memory leak occurs every time the client disconnects.
The only time when multiple Xvfb servers are useful is if you have several processors available in the machine (for example, 8 cores) and your script is heavy. To make sure this is the case, plug in many instances of your script and check top to find out what Xvfb CPU usage is. If it is 100%, you can use additional instances of Xvfb.
jterrace
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