un séptimo piso

Hola John

You can say both in Spanish: "Vivo en UN séptimo piso" and "Vivo en EL séptimo piso".

You'd use "un" when you are talking more generally: "on a seventh floor" (in a block of flats) and you'd use "el" when you are being more specific; Imagine you are saying this to someone at  the street and you have the block of flats where you live in front of you. You'd say "Vivo en el séptimo piso" looking and pointing at the top of the building. 

"Vivo en la/una séptima planta" is also correct (but feminine in this case)

Saludos