There are 6 Robot Masters in the original Mega Man game. They are Cut Man, Guts Man, Ice Man, Bomb Man, Fire Man, and Elec Man.
Released by Capcom in 1987 for the NES and Famicom, the game established the series' signature structure. Players select from six stages in any order, defeat the Robot Master at each stage's end, and copy that boss's special weapon. Each boss has a weakness based on another Robot Master's weapon, encouraging players to experiment with the stage order.
A common mix-up comes from later games, which usually feature eight Robot Masters in their opening stage selection. Mega Man 2 onward expanded the standard lineup to eight bosses, while the original game has only six. The Wily fortress also contains additional bosses and rematches, but those do not change the number of initial Robot Masters.