As far as I understand, it is commonplace for editors to invite M reviewers when N < M would suffice. After N reports arrive, they take a decision and inform the authors. What happens to the N+1st, N+2nd, ... referee reports that arrive after the decision has been taken?
As a referee, I have only been told “thanks, we won’t need your report anymore” a very small number of times, and as an author I’ve never been told “look, there’s one more report for you”. So I wonder what happens in practice.
EDIT: I have in mind journals, not conferences. A conference is more driven by deadlines, probably.