How many simultaneous visitors Craft CMS can handle?
Is CraftCMS (v3) realistically capable of handling around 10.000 logged in visitors simultaneously?


Generally speaking, yes. Depending on the specifics of the site and what the users are doing, you might have to make some environmental accommodations.

As always with stuff like this, so much of it depends on things like the hosting architecture (load balancers, web servers, hardware specs, session/cache storage, caching) as well as Craft's information architecture, template logic, plugins, caching, etc.