Is this a mistake in Harry Potter and the Deathly Hallows? -Spoiler!?

Ron is just talking about Horcruxes. The trio believes that Voldemort has split himself into seven parts: 6 Horcruxes and the seventh part is the part of his soul that still resides in his body. So Ron thinks they only have to destroy 3 more Horcruxes before they can defeat Voldemort.