Free for All - S1-E4
Trivia: This episode's writing credit reads "Paddy Fitz." This was one of many pseudonyms Patrick McGoohan used in writing, directing and producing most of the series himself. "Fitz" was borrowed from his mother's maiden name, Fitzpatrick.
Answer: It's even more obvious than you think, you know who number 1 is in the very first episode. When 2 replies to the question "who is #1?" Change the way he answers from you are number one (in the monotone or accented answer to, "You are, number 6. The comma gives you the answer. #6 is #1. It's the tone of the answer.