Timely review process by reviewers
I disagree slightly with Peter Jansson's answer, which says that you should only search for new reviewers if the ones that you have assigned fail to produce.  In my experience, there is a decent chance that at least one of your reviewers will fail to produce a review in a timely fashion, so it is better to plan in advance.

As such, I strongly recommend recruiting slightly more reviewers than are needed for a paper to receive proper review.  For example, if a journal requires three reviewers, then you can recruit four.  Invite even more than that initially, since it is virtually certain some of the reviewers you initially invite will decline or fail to respond.  If, by some chance, all of the reviewers return in a timely fashion, then it's generally not a problem for the authors to get some extra feedback.

Further, never tell a reviewer that you don't need their services until you've got enough reviews to make a decision on a paper.  I've had the experience where a newly recruited reviewer fails while a long-delayed reviewer eventually actually turns up with a review.  If you dismiss the "presumed failed" reviewer before you've got sufficient reviews in hand, you cut off the possibility of such nice surprises.