The man who becomes Vecna was assigned the test-subject number One at Hawkins Lab. His original name is Henry Creel, and he is also identified as Subject 001 or 001 before becoming the supernatural villain Vecna.
Dr. Martin Brenner took Henry into his program after Henry used psychic abilities to murder his mother and sister and frame his father, Victor Creel. Brenner made Henry the first subject in an effort to reproduce his powers in other children. Henry later worked as an orderly at Hawkins Lab, where he manipulated Eleven into removing the device that suppressed his abilities.
Once free, One massacred the other children and staff members at the laboratory. Eleven confronted him and defeated him, sending him into another dimension, where his body and powers were transformed into Vecna. The number is commonly expressed in two forms: "One" is the spoken name, while "001" is the numerical designation. They refer to the same person, so the stored answer is the canonical answer rather than a different subject such as Ten or Two.