HOW CONCURRENT USERS WORK FOR LOADUI?

I have 5 users need login, then execute same API call, when I select 1 virtual user to run this load testing for 10 min, from log, I can see it loop 5 users, not only first user. api call is passed, when I change to 5 virtual uses, api calls will have a lot errors because it randomly select user login. how the concurrent users work in loadUI?.