In system-generated games, there is a limit of 500 starships that can be built. (This limit can be changed for Custom games.) This "ship limit" is imposed to prevent the game from becoming too big to easily play while encouraging players to begin fighting in order to build more starships.
Under the classic Priority Build Points system, this is a hard limit; as a result, only ships numbered in the range from 001 and 500 (or whatever the ship limit happens to be in the game) can exist. Under the newer Priority Queue system, this is a soft limit, and allows priority builds to occur as long as the building player has enough Priority Points to pay for them (or until the programmatic ship number limit of 999 is reached).
It is worthy of note that in sectors where the Horwasp Plague are engaged, much of their new ship construction is neither limited by the Ship Limit nor limits the vessels of other factions.