You seem to have a variation of issues.
First of all: Ensure that your postgresql server (if being used) allows more connections than the default of 100. For this have a look at it’s configuration file, you should find
max_connections there. Raise it to at least 500. Don’t forget to restart your service.
Also, too many open files is an indicator that your system limits to current opened files. This can have various reasons, but is more of a OS issue than a Zammad issue. I have never seen this on a system with a single Zammad installation. If you have several services beside Zammad on the same machine, Zammad might just push you over this limit.
Spoiler alert: This issue will appear within 72hours (my guess). Rebooting does not solve it, raise the limits on your OS if needed. This may also cause your OS do not allow processes to run as expected and degrade service quality greatly.
Further information on possible tweaking can be found here:
Personally I think you want to touch your web concurrency. If you also need more schedulers for just 10 agents, you proberbly have tons of overviews which might not be what you really want. (also have a look at https://admin-docs.zammad.org/en/latest/manage/overviews.html).