No, nor can anyone else, but I can tell you how to think more clearly about the question.
In some senses, your Apache MaxClients settings is how many concurrent users you can "handle"; likely you'll want to increase it, and the questions there are whether you have enough memory to run that many Apache processes without hitting swap (which, of course, depends on how big a memory footprint your Apache processes actually have under your build and configuration) and whether your machine and the operations it's performing are fast enough that memory is even a worry.
The speed question leads into your polling script's performance: if it takes 0.01 seconds to run, and every user is hitting it once a second, and you have 4 CPUs, then the most people you could theoretically possibly have without jobs starting to wait for each other is 400. (In practice it'll be less than that because the jobs don't come in in a perfectly orderly line, parallelization isn't perfect, and the machine has other things to do too.)
But, y'know, since we have these gracefully degrading systems, that isn't really an important number; the critical thing is how many people it takes to make the system degrade "too much" for your subjective purposes. It it okay if someone hitting your poller waits 0.1 seconds for it to come back? 0.2 seconds? 3 seconds? Somewhere in there is a line where you'll say "okay, no, this performance is too bad". The number of users you have at that point is your capacity.
