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.
The image of the instance was made with the Duplicator plugin. Download the zip file, unzip it, and follow Duplicator’s instructions to install it.
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.