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.

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

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