This is the number that was assigned to Alex in A Clockwork Orange while he was stationed in Staja "(State Jail, that is)" Number 84F. It was sewn on his "heighth [sic] of prison fashion" uniform above his groody (breast) part above the "old tick-tocker" and on the back. 6655321 is how Alex is referred to throughout his stay by the prison personnel, including the prison charlie (chaplain).
After Alex is chosen for the Ludovico Technique, he is again referred to as "Alex" and never as "6655321" again.