A good question! Sunspots form when magnetic flux tubes rise inside the Sun and break through the photosphere. The magnetic field lines of a flux tube rise from one spot and they go back inside through the other spot (see the attached figure). That is why sunspots are usually seen as pairs. Strong magnetic fields tend to emerge in certain regions, called active regions. Active regions may have several sunspots and they are the also the places where most solar eruptions like flares and coronal mass ejections occur. I suppose single sunspots occur in the early or decay phase of the sunspot pair. Also sometimes a group of sunspot may appear like one big sunspot due to low resolution.