Hello,
No problem, it's a pleasure to help.
1. You can run multiple DayZ servers by changing the ports that your servers will use. You can use OmegaManager to do this, it will allow you to create and manage multiple DayZ servers.
2. If you only have the Ryzen 5800X, you can deal with this. It shouldn't be too much of a hassle. On an 8-core processor, it is best to limit yourself to running 4 DayZ servers at most. Beyond that, there is a risk of a strong performance degradation.
3. Unfortunately, it is not possible to test performance via a script that simulates the presence of players. However, if your servers are running on a base of 15,000 FPS servers (it is rare to go beyond this on DayZ, although technically possible [1]), it should perform well with 80 players. It is important to ensure that FPS servers are not degraded by the presence of mods. For example, the AI bots in DayZ Expansion tend to be very greedy: you have to be careful on this point.
4. The data transferred between the server and the players is not very important, it will be enough for 80 players. (However, we are talking about upload speed here)
[1]: I use a Ryzen 9 7950X3D to host my clients. If I limit the number of DayZ servers to 8, I can easily reach 21,000 - 28,000 FPS servers on average across all servers. Both in vanilla and with many mods.
Good continuation to you!