The Ambassador Class also had 1 Impulse Port, on the neck. The number of impulse ports on a ship, apparently, is not conducive to the ship's size. The Sovereign Class, for example, is longer than the Galaxy Class, and roughly the same mass. But has half as many decks. And yet, it only has two impulse engines on the saucer. The Galaxy Class had three, but only 1 was active at all times. The two on the saucer were not in operation unless the saucer was separated from the rest of the ship.
However, the SIZE of the Impulse Ports could be important, as the Impulse Engines do provide the sublight thrust of the vessel. The smaller the port/the bigger the ship, the more sluggish the vessel would be at impulse speed.