We often get asked how many users Prosody can handle. Or we get asked for benchmarks. We are reluctant to answer either of these questions (as any honest project should be). Here is where we try to explain why…
There is no correct answer to the "How many users?" question. It depends on many things. Here are some (not all) of the possible things that could affect how many users Prosody can handle:
For example, Prosody can handle many more users if you disable all modules, all the users have empty rosters, don't generate any traffic, and don't use encryption or compression. Obviously this scenario is not very realistic, as an XMPP server configured this way would not be very secure or very useful. Yet some other server projects have published benchmarks demonstrating their impressive scalability under just these circumstances.
We'd like to provide a realistic and honest figure, but there are so many different configurations, and every setup is different. It's impossible to just choose one number.
For similar reasons. Because every setup is different, benchmarks from two different setups cannot be compared. Even different benchmark tools on the same system will typically produce massively different results. Running benchmarks is a tricky operation. To be done correctly you have to be very careful to run them on a clean system, preferably not in a virtual environment or anywhere that CPU could be "stolen". You have to ensure your CPU isn't doing automatic frequency scaling, and so on.
This all makes publishing benchmarks next to useless, and harmful in fact (because people will naturally want to compare them to other projects). This would make Prosody incorrectly look either better or worse than other projects.
I encourage you to do your own tests. Also, I wasn't entirely truthful with you. We do have some benchmarks (produced and hosted by a Prosody developer). They show the amount of RAM and CPU required to connect up to 1000 idle users. This gives a rough baseline for memory usage, but realistic configurations will almost certainly be higher than this.