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.
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.
Data is transmitted in packets. Each packet has a packet header, and a packet payload. The total data transmitted is the header + payload.
If you're transmitting smaller packet sizes, it means your header is a larger percentage of the total packet size.
Measuring in megabits is the ISP telling you "look, your connection is good for X amount of data. How you choose to use that data is up to you. If you want more of it going to your packet headers instead of your payload, fine." A bit is a bit is a bit to your ISP.
@Moneo@SigHunter Networking came to be when there were lots of different implementations of a 'byte'. The PDP-10 was prevalent at the time the internet was being developed for example, which supported variable byte lengths of up to 36-bits per byte.
Network protocols had to support every device regardless of its byte size, so protocol specifications settled on bits as the lowest common unit size, while referring to 8-bit fields as 'octets' before 8-bit became the de facto standard byte length.