Number Two was an informal title granted to those Imperial Viceroys who never served as Prime Minister or asQuasi-Sovereign.  This was particularly true when the Star Imperium lacked a ruling Imperial Sovereign and the powers of the Imperial Sovereign were exercised by the Council of Twelve during that coouncil's various regencies. 

When there was a ruling Imperial Sovereign, especially after the formation of the Council of Ministers in Year 3465, the Viceroys who did not serve as Prime Minister served as the number two member of the Council of Ministers.  In this case, he or she gained the informal title of Number Three.