When you create a test run, your select which test cases are included in that test run. So you can have 100 test cases, but only include 50 in each test run. That is precisely the purpose of the "Start a test run" screen, to help you do that :-)

Click on "Navigate" in the header or use CTRL+K on your keyboard.

This is done

There is now a "Copy" button for each test case, next to the delete and expand buttons

This is a personal opinon, some people prefer usernames. You can also use your email to login, it works!