Vitruvius' De Architectura is the oldest and most influential book on the subject. There, he attempts to establish a canon for architectural proportion and symmetry, including the characterization of a temple by the number of columns on its principal façade.
A temple could be tetrastyle (having four columns on the front), hexastyle (six, Temples of Saturn, Vespasian, Concord, Divine Julius), octastyle (eight, Temple of Castor and Pollux), or decastyle (ten, Temple of Venus and Rome).
As one can see in the remarkable watercolor by Constant Moyaux (1865), the Temple of Concord has six columns and so is hexastyle. Situated against the Tabularium, the restrictions of the site required the cella of the temple to be almost twice as wide as it is deep, as is the pronaos, the columnar porch in front of the cella that is approached by the stairs.