Selfhosted

Just look at the bit rate of what you are streaming and multiply it by 3 then add a little extra for overhead.

I have 35mbps upload from the ISP, and limit each stream to 8mbps. This covers direct streaming all my 1080p content and a 4K transcode as needed.

Are you transcoding?

4mbit per client for 1080 is generally a workable minimum for the average casual watcher if you have H265 compatible clients (and a decent encoder, like a modern intel CPU for example), 6 - 8mbit per client if its H264 only.

Remember that the bitrate to quality curve for live transcoding isn't as good as a slow, non-real-time encode done the brute force way on a CPU. so if you have a few videos that look great at 4mbit, dont assume your own transcodes will look quite that nice, you're using a GPU to get it done as quickly as possible, with acceptable quality, not as slowly and carefully as possible for the best compression.

My family is very satisfied with 6 mbit/s per stream. Some HEVC, most H264. They see it as high quality. 3 Streams would be 18 to 20 Mbit/s

How expensive is internet? If its cheap go overkill and don't worry about it.

It depends on many factors, not 4k Is one thing but how if your okay with 480p you can probably get away with 25mbps upload and still hands 3 or 4 streams, Of course you also need to consider the need to transcode the streams.

I would recommend at least 100mbps but its possible to manage with less

I don't have a jellyfin server but 1MB/s (8mbps) for each person watching 1080p (3.6Gb per hour of content for each file) seems reasonable. ~3MB/s (24mbps) upload and as much download should work.