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.

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.