First of all, let's be clear and state that metronomes are not really short people who live in a large city.  They are clicky things that help you to keep track of the time while you are playing music.  Some metronomes do not click - they flash. My wife has one of those, though her metronome also clicks - unless the batteries run down and then it does neither.

It is only necessary to have one metronome to keep track of time, but I suppose a second one as a backup would be good should your batteries ever run down.  It would be complete over kill to have, let's say, 5 metronomes.  Unless, of course, you wanted to make a video showing how to get them all ticking at the same time.  But who would ever do that?  This guy would - that's who.