moodlemuse The scalability of video content is a broader topic than MoodleBox or even Moodle itself. The cause is buried in the technology. When a web server transmits a video it just relays a file, the video, one-to-one through the web server. The web (HTTP, HTTPS) was not designed for simple file delivery, and as a result are inefficient doing that. There are specialized servers, https://en.wikipedia.org/wiki/Video_server, for this.
I don't have the numbers on how many videos can a MoodleBox deliver in parallel. A theoretical maximum would be, say 300 Mbit/s if all devices are 802.11n divided by 3 Mbit/s if HD, resulting 100. In practice it will be much less. If nothing else the MoodleBox can communicate only with roughly 20 wireless clients at a given time. This post https://discuss.moodlebox.net/d/313-moodlebox-mit-powerbank/8 quoted 30 but that is "too good to be true". It wouldn't be too difficult to do a field test. As I wrote in another discussion https://discuss.moodlebox.net/d/315-moodlebox-on-a-pi-400/8, I don't expect this number to be different for a RPi 3 B or 3 B+. If you do any tests, please publish the numbers here.
Yes, delegating the video "streaming" (a web server does not really stream, the video gets progressively downloaded) to a video server will relieve the web server in the MoodleBox. Whether that will increase the number of clients supported depends on where the bottleneck actually is. Say, if the limit comes from the wireless circuitry itself, then a separate video server will not give you more clients.
Finally, running the video server in the MoodleBox RPi itself, that is a hard question. Firstly, it makes any sense only if the load of video transfer the limiting factor, not the wireless circuit as explained above. Secondly, if it is the case, the video server will relieve the web server but bring a new load on the OS (processing, RAM). Reading the file in the SD card remains the same. To put it another way, at the current state of RPi technology, the I/O throughput of the SD card and the data rate of wireless are in the same range. So, can't say, which one comes first. As always, the answer to where exactly is the bottleneck is critical. So your own tests and benchmarks are the best.