Maximum Users

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Jellyfin is meant as a personal media server. While theoretically this is possible, you're likely to encounter MANY bottlenecks along the way, including drive I/O, network throughput, ISP speed, and likely you'd garner some attention streaming to 4k people at the same time.

The overhead for non-transcoded media is minimal, though there is definitely a limit there as well. Jellyfin is not meant as a commercial solution and does not cater to that sort of feature set.