Hola Mark

You could also use "Vivo en el séptimo piso" but choosing the definite article "el" would imply talking about a specific 7th floor, i.e. you'd probably be speaking to someone and pointing at the specific block of flats where you live as in saying "I live on the 7th floor of this building here." - When we use the indefinite article "Vivo en un séptimo piso" we're talking more "generally" without meaning "in this or that building", but simply saying that you live on "a" 7th floor somewhere. 

I hope this clarifies it.

Saludos

Inma