I'm assuming that you're talking about
Rotala rotundifolia, which is quite common in the hobby.
R. indica is really something else.
http://www.aquaticplantcentral.com/f...us&spec=Rotala
Anyway, as with many stem plants, the number of leaves per node is variable. Nodes may have two, three or even four leaves.
Rotala sp. 'green', which may or may not be a variety of
R. rotundifolia, always has two.