As far as I’m aware we do not limit the maximum concurrent number of logged in users.
The ressource usuage of each user highly depends on what he’s doing. Users with 1 ticket-tabs open will need less ressources then users with 30 ticket-tabs.
Of course you will slow down the application if your hardware can’t keep up with the workload. I haven’t seen a crashing Zammad (because of that reason) yet, but it will be reeaaallyyy slow if this happens (up to throwing 500 Errors).
I think @anon29869905 can give a better heads up on this.
Beside the following page will give a good hint on what you’d need to do to have more users:
https://docs.zammad.org/en/latest/prerequisites-hardware.html
I don’t know what you’re planning, but it sounds rather enormous.