6 World Trade Center had 8 total floors, starting at the street level.
The problem is that the actual
visible building has only 7 visible floors starting at the Plaza Level, but there is an additional lower level on the Concourse Level. While this section is at the same level as the Mall at the World Trade Center, it isn't actually connected to the mall in any way, and instead originally had a direct entrance/exit on Vesey St., as can be seen in these images:
You can see the seven upper floors, and the lower street level doors of 6 WTC.