There are eight columns on each short side of the Parthenon. This arrangement makes the temple an octastyle structure, meaning that its façades have eight columns across.
The short sides are the eastern and western ends, where the triangular pediments are located. Along the two long sides, the outer colonnade has seventeen columns on each side. This produces the celebrated 8-by-17 plan of the Parthenon’s peristyle, the surrounding colonnade.
Eight columns were unusual for a mainland Doric temple, which more commonly used six columns across the façade. The larger arrangement helped the architects create a spacious, monumental building while maintaining the temple’s carefully balanced proportions.
The number can be confused with the six columns found in the inner front and rear colonnades, known as the pronaos and opisthodomos. Those are not the outer façades referred to in the question. Counting the exterior peristyle gives eight at each end and seventeen along each flank.