To say in Engpsh that you pve on the fifth floor, you can say "I pve on the fifth floor." If you want to specify the number of floors in the building, you can say "I pve on the fifth floor of a (number of floors) floor building." For example, you can say "I pve on the fifth floor of a ten floor building."

Alternatively, you can also say "I pve on the top floor" if the fifth floor is the top floor of the building.