According to the Star Trek: The Next Generation Technical Manual, the Enterprise-D is equipped with six lateral sensor arrays. The Primary Hull has forward-facing, port-facing and starboard-facing sensor arrays, while the Secondary (Engineering) Hull has aft-facing, port-facing and starboard-facing sensor arrays.
The Galaxy Class Enterprise features one of the most sophisticated and flexible sensor packages ever developed for a Federation starship. These sensors make the Enterprise one of the most capable scientific research vessels ever built.
There are three primary sensor systems aboard the Enterprise. The first is the long-range sensor array located at the front of the Engineering Hull. This package of high-power devices is designed to sweep far ahead of the ship's flight path to gather navigational and scientific information.
The second major sensor group is the lateral arrays. These include the forward, port and starboard arrays on the rim of the Primary Hull, as well as the port, starboard and aft arrays on the Secondary Hull. Additionally, there are smaller upper and lower sensors arrays located near Decks 2 and 16 on the Primary Hull, providing coverage in the lateral arrays' blind spots.
The final major group is the navigational sensors. These dedicated sensors are tied directly into the ship's Flight Control systems and are used to determine the ship's location and velocity. They are located on the forward, upper port, upper starboard, aft, and upper and lower arrays.
In addition, there are several packages of special-purpose and engineering sensors such as the subspace flow sensors located at various points on the ship's skin.
See the diagrams below for the precise locations of the sensor arrays.