Here’s a WordPress instance with 100,000 fake users made with the DemoPress plugin. It also contains fake posts, pages, and comments. If you need to test a plugin or theme with a large site, this may help you.
Here’s the zip file. https://www.plumislandmedia.net/downloads/wordpress_5_9_3_100k_users.zip
The administrator username is administrator. The password is thousands.