Hilchos Lashon Hara, chapter 2, section 1

It is forbidden to speak lashon hara about another person, even if what is said is true, and even if only one person hears it.

The more people who hear it, the bigger the speaker's sin. There are two reasons. The person spoken about is shamed more, because more people know the bad thing. And the speaker causes more people to sin by listening to lashon hara.