Yes, I think it's a minimum of 1 wave length per leg, but multiple wave 
lengths mean more gain.

Most people would optimize their rhombic for a given band, say 20 or 15 
meters, and then use it on any of the other bands where they could get 
it to tune.

On the main band, the strongest gain is in two directions. I remember 
hearing W7LUN say that his favored Indianapolis, and Melbourne, 
Australia on 15 meters.

There is also a terminated rhombic, which has a resistance at the end 
opposite the feed point. That changes both the gain and the 
directivity, but I don't know exactly in what ways.

The legendary Don Wallace, the original holder of W6AM, had 100 foot 
tall utility type wooden poles that held multiple rhombics, each 
favoring different directions.

His family made a mint worth of money when that system was dismantled 
and the property sold to developers.

-- 
Mike Duke, K5XU