Brainteaser - Shaking Hands

At a sophisticated art gallery opening, Jacob and Emily joined 4 other couples. As it's customary in the art world, individuals greeted and shook hands with those they hadn’t met before. Emily, being an observant person, made an interesting observation: aside from her, everyone else seemed to have shaken hands a different number of times.

Using Emily's observation, can you deduce how many people Jacob shook hands with at the art gallery?