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Keeping it real.
There is no such limit set in software that you will be able to only support XXXX number of user at a time.
We use NODE.JS, SOCKET.io to update auctions real time it keeps server resuurces free most of time only update when it's actually needed.
When your trafic increase we have solution to handle it using load balancer, different server for front end, different server for MYSQL etc.