upper limit on concurrent editing?

The most of editing process (from loading editor page to saving post) happens in browser and doesn’t involve server.

The one exceptions is auto save feature – post will be quietly saved by JavaScript in background, every minute by default. This might cause performance issue if you have considerably large posts, considerably many editors and poorly performing database.

Auto-save interval can be customized by defining AUTOSAVE_INTERVAL constant in wp-config.php.