There is no technical limit in terms of "how many documents can be uploaded" and the question "how many users can use it concurrently" can always be answered with "it depends on what resources you assign to the Alfresco system" and "it depends on the use cases that define their patterns of access". E.g. there is a significant difference between hundreds of users only logging on to Alfresco and looking at some pre-cached overview, or the same amount of users actually doing something with the system, changing document metadata, uploading new files, processing workflow, generating reports...
