No matter how many side (ground) electrodes there are, only one spark takes place during the high voltage discharge for the power stroke.
Sparkplugs for cars usually have one electrode. But other designs have been marketed, notably those with two and three ground electrodes. The proper configuration is actually FOUR side electrodes, since all aircraft engine plugs have FOUR. The purpose of the multiple electrodes is to even out the erosion, thus extending the life of the plugs and assuring a fairly constant gap throughout its useful life. It is not to give "more power".
The reason why FOUR electrode plugs are not used on cars is cost. They cost over ten dollars each. And for planes, its even worse, since there are TWO PLUGS per cylinder, each driven by an independent, self-powered magneto ignition. With two plugs and two mags, you actually get TWO SPARKS at the same time (actually staggered by 5 degrees or so). Now if only Chrysler would put that in their cars instead of those idiotic computers and their 100 wire connectors to go wrong. Then, they could resume bragging about how Chrysler stands for engineering.