Sung Kil Moon - 7 fights (against Khoakor too, very legit) Joichiro Tatsuyoshi - 8 fights (amazing performance)
Can you please clarify what you mean. Do you mean who took the fewest fights to became world champ... … or are you looking for the world champ who retired with the fewest fights under his belt?
You are not going to beat the original champ, who by definition won the title on their debut, so that would be for the lineage of the modern era, James Figg.