According to Genesis, Noah's Ark had three decks.
Genesis 6:16 describes the Ark as having lower, second, and third decks. The same verse also mentions a window or opening, a door in the side, and a covering, though translations differ in how they render some architectural terms.
The three-level design fits the narrative's description of a vessel large enough to contain Noah's family, animals, and stored provisions. It does not provide a detailed engineering plan, so later reconstructions necessarily make assumptions about ramps, compartments, ventilation, and animal housing.
The number of decks is separate from the Ark's overall dimensions. Genesis gives three decks and also states 300 cubits long, 50 cubits wide, and 30 cubits high.