What happens to surplus referee reports?
From my experience, things happen differently. Comments do highlight it correctly.

Initially, N reviews are solicited by the editor. If they disagree (or, I have to assume, some reviewers miss their deadlines), more reviews are solicited. I've been reviewer #5 few times.

Typical values for N are: 


2, with third as a tie-break – a typical grant application review scheme;
3, with 4 or even 5 as a tie-break - seen more often with journal papers.