Bandwidth

Media files are usually large files.

Watching/Streaming media across the internet is basically just downloading the file but at a speed needed for playback and not saving it to disk, just displaying and forgetting.

You have to have an internet connection fast enough to stream the file at the needed speed.

There is no live transcoding or any modification of the media files.
It's just a simple file streamed directly from the server to the clients.

If you want to save on bandwidth you have to transcode the file to a lower resolution and add it on the server beforehand.

Example

Say you have a 4k movie you want to watch with 4 other friends.

Say the movie has an average bitrate of 25 Mbps (that's "Mega bits per second").
So to be able to watch it you should have an internet connection capable of at least that speed.

The movie doesn't always have the same bitrate so there are moments where it is higher and lower, and there is buffering in addition to that.

That explains the client side (you and your friends) needs for bandwidth.

The server needs all of your bandwidth combined.
Since it is sending the file to all of you at the same time, it needs enough speed to do so.

Servers usually come with 1 Gb/s connections available, that is 1000 Mbps.
So theoretically your watch group could be around 40 people in total to watch that 4k movie at the same time.

Some servers offer 10 Gb/s connections, which would x10 your throughput theoretically allowing for around 400 people.
(I have not tested this yet with such a big group, issues may occur)

Depending on where and how you have your server set up, be careful not to go over your allowed data cap.
That could lead to additional costs.