A Sleeper Ship is an extremely large craft designed and developed by the Western Alliance. Five (officially four) of the Sleeper Ships reached the Sirius Sector: the Liberty, the Rheinland, the Kusari, and the Bretonia, as well as the Hispania. Seven were built: the French and Italian fleets were wiped out in the initial surprise attack which started the Alliance/Coalition War. Likewise, the Italian and French Sleeper Ships were apparently destroyed during the escape (perhaps due to lack of dedicated escorts).
The interior of the sleeper ship as shown in the opening cinematic of Freelancer shows several decks lined with nothing but sleep-pods filled with Alliance refugees. Obviously, each ship also contained a small stay-awake crew to fly the ship, plus supplies, Earth-lifeforms (including plants, bacteria, and animals such as fish), and other colonization materials. It is difficult to estimate how many passengers were on board each sleeper ship at the time of the Exodus.
The Liberty still stands on Planet Manhattan near the starport, apparently refurbished into a building. The Hispania floats in a debris field in the Omicron Alpha system near Planet Malta. Kusari, Bretonia, and Rheinland all cannibalized their Sleeper Ships during the initial colonization of Sirius, and have not bothered rebuilding a "vanity model" like Liberty has.