Yes, mergers were more common at earlier times.
But in fact, even with mergers, it's exceedingly difficult to get supermassive black holes to merge in the first place. Because they are tiny compared to the sizes of the galaxies they inhabit, they have to somehow be pushed close enough that they start orbiting each other and radiate away orbital energy in the form of gravitational waves. Only then can they hope to merge into a single black hole.
It's actually so difficult that one popular explanation for the existence of supermassive black holes is that they are the peaks of the primordial density perturbations that collapsed into supermassive black holes in the very early universe.