As many as the system / infrastructure configuration allows and you have requests / users hitting the system. The question is a bit pointless without specifying a bit more detail / context to your request. Are you interested in any software limits, or are you wondering which parameters / configuration affect the number?
We have a use case where 3000 users could log in to the system. So what is the number which alfresco can support?
Again, technically only limited by the resources your system has available. There is no logical limit in any way. If your server has ~100 CPU cores available, you could support ~100 users at the same time - assuming each user is executing their request at the same time / without pause etc.
Since users won't typically work with a system at the same time, have different patterns of usage etc., things become quite hard to estimate unless you know your specific use case quite well. I had a customer use Alfresco to provide a platform to ~70.000 users with only two servers of 8 CPU cores each - but only <= 10% of all users would access Alfresco on any single day.
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