The number tattooed on Henry Creel's arm at Hawkins Lab was 001.
The tattoo identifies Henry as Subject 001, or “One,” the first person in Dr. Martin Brenner's series of Hawkins Laboratory test subjects. Henry was not an ordinary child recruited into the program: he already possessed powerful psychic abilities before Brenner studied him.
Henry later became the orderly who manipulated Eleven inside the laboratory. After she removed the device suppressing his powers, he massacred the other test subjects. Eleven defeated him and banished him to another dimension, where his body and abilities were transformed into the villain Vecna.
The number is easy to confuse with Eleven's tattoo, which reads 011. Other numbers, including 008, belong to different test subjects. The 001 marking is the key clue linking Henry Creel, the laboratory's first subject, and Vecna.