Wordpress How many concurrent logged in

Hard to know logged in users is highly dependent on how your wordpress install is made up i.e. what plugins are used, size of the wordpress database/data/blog blogs, and specific hardware configuration of the server. For example, huge difference for non-cacheable logged in users for resource usage between a Wordpress install with no plugins versus Wordpress install with 10 blog posts and displaying latest 10 blog posts via a widget versus a Wordpress install with 100s of plugins and 100,000 of blog posts trying to display the same latest 10 blog posts via a widget.

It's harder also because wordpress logged in users can't use full HTML page guest caching or any advance page level caches so it really comes down to the server and it configuration. Only way to know is test it yourself and then monitor and tune your server settings for your loads (particularly PHP-FPM) and upgrade or download servers as needed

Ensure your Centmin Mod takes advantage of additional performance boosting options outlined at https://centminmod.com/perf/ - particularly enabling PHP 7+ Profile Guided Optimizations and enabling Wordpress specific PGO training - example and benchmarks outlined at PHP - PHP 7.3 vs 7.2 vs 7.1 vs 7.0 (PHP-FPM) Benchmarks

And of course use PHP 7.3 where possible.