I'm two
Sung from the point of view of the band itself, it became just two people, Tyler Joseph and Josh Dun when Nick Thomas and Chris Salih left in 2011.
Additionally, this can be seen as a double entendre if you include the multiple implications throughout the album that Tyler has two halves, one good and one bad. We can see this more clearly explained later upon the release of Blurryface which divulges the concept in a unique way.