The Number of users a Wordpress Web server can handle simultaneously is a function of the Server Capacity/Specification/Configuration, more specifically the size of your WordPress user’s visit process and the Server Ram has a role to play in this number.
There is a Crude formula that can give you this
X = Process Memory usage Size
Y = Minimum Memory required for you webserver to function (500MB in case of APache)
Z = Total RAM available
Number of users at a time (MaxRequestWorker) = (Z - Y)/X
So for a 10000 MB (10GB) Server and 1MB process size
(10000 - 500)/1 = 9,500 Users.
This above has lots of assumptions like Apache is the only service running on that server and that the server uses 500MB and process is 1mb
This is essentially a theoretical limit which may be different in reality, however it would give you a rough idea of what to expect.
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