How many users max?

I honestly don’t know. But you can try it yourself when v2.10 is released. That version has a function to automatically create test users.

The calculation itself splits the work in small parts, so I won’t expect errors there. It will just take very long to complete.

But I can already tell you that some views (e.g. the default ranking list) won’t work well with large user sets. Even a browser will break if you show a list of 50.000 users without pagination.

Is there any other method to add test users other than the CLI method (I get an error there)?

There are also plugins that can add test content and users. But please beware that it can be difficult to delete test data. If you are not careful, you delete too much. So I recommend creating a backup that you can restore before you start testing.

Once you have the users, you can create an SQL script to copy your predictions to all other users (or randomize the values). Or you can write a PHP script to do this.