In fact all volcanoes could be complex volcanoes since all of them are made up of multiple flows, ash layers, domes, cones, etc. in varying amounts.

     However, when we call a volcano a complex volcano it is because we mean the "system" of those volcanoes is not "simple".  Caldera complexes for instance have often got a large caldera with many subsidiary vents and deposits, some of which could be considered "volcanoes" in their own right.

     Frankly speaking, a volcano that consists of a complex of two or more vents is reckoned as a compound or complex volcano.