Centipedes, which are members of the class Chilopoda, always have an uneven number of pairs of walking legs, ranging from 15 to more than 171. The true centipedes (order Scolopendromorpha) have either 21 or 23 pairs of legs. Common house centipedes (Scutigera coleoptrato) have 15 pairs of legs.
Sources: Grzimek's Animal Life Encyclopedia, vol. 1, pp. 512-13; Palmer, Ephraim Laurence. Field Book of Natural History, 2nd ed., p. 441.