It's true that most patients have one or two levels fused for most spine problems. In the aging adult degeneration, stenosis (narrowing of the spinal canal), and disc herniation are the most common reasons for spinal fusion.
Fewer patients have four, five, or more fusions. Some conditions such as scoliosis may require many levels to be fused to gain the stability needed. In cases of multiple level fusions, rods are often used along with bone grafting to help support the spine as it fuses.
Further problems can occur if the spine is unstable above or below the level of a fusion. This is true for any number of fusions whether single, double, or more. The surgeon wouldn't recommend a five-level fusion if the spine was stable at any of those levels.