Kane had been studying a nearby solar system called Trappist-1, which has three Earth-like planets in its habitable zone.

“This made me wonder about the maximum number of habitable planets it’s possible for a star to have, and why our star only has one,” Kane said. “It didn’t seem fair!”

His team created a model system in which they simulated planets of various sizes orbiting their stars. An algorithm accounted for gravitational forces and helped test how the planets interacted with each other over millions of years.

They found it is possible for some stars to support as many as seven, and that a star like our sun could potentially support six planets with liquid water.
Gas giants can be a hinderance due to strong gravitational disturbances:

Why then does our solar system only have one habitable planet if it is capable of supporting six? It helps if the planets’ movement is circular rather than oval or irregular, minimizing any close contact and maintain stable orbits.

Kane also suspects Jupiter, which has a mass two-and-a-half times that of all the other planets in the solar system combined, limited our system’s habitability.
Full article (study linked within)