There are only three riders who managed to win eight stages during a single Tour de France edition. One of them is Eddy Merckx, of course. He achieved it twice, in 1970 and 1974. The other two cyclists are Charles Pelissier (1930) and Freddy Maertens (1976).
Charles Pelissier (1930)
Charles Pelissier, the youngest brother in the famous Pelissier family, who was compared to the movie star Rudolf Valentino in his book "1926" by the German scholar Hans-Ulrich Gumbrecht, dominated in the flat stages in Tour de France 1930. This was the first time national teams competed the race, and the final winner was also a Frenchman, André Leducq.
Pelisser not only won 8 stages, but finished second 7 times and third 3 times. That means, he was in top3 in 18 of 21 stages. Not surprisingly, he lost significant time in the mountain stages, once in the Pyreness and twice in the Alps. But he also managed to win a stage included Port d'Aspet and Puymorens. (It was a buch finish, several cyclists arrived with the same time.)
Charles Pelissier won a total of 16 Tour de France stages during his career. Although his other participations in the French grand tour were less spectacular, he still managed to win 5 stages in 1931. Also, he wore the yellow jersey for a few days in that year.
Eddy Merckx (1970 and 1974)
The edition in 1970 was Merckx's second Tour de France victory, the one in 1974 was his fifth (and last).
In 1970 there were 5 individual time trials in the program (and a team time trial too) which makes bit more understandable, why a cyclist was able to dominate the race this way in the modern era of road cycling, even when this cyclist was Eddy Merckx. However, he won "only" 4 of the 5 individual time trials. The other four stage victories came from a flat stage and 3 mountain stages, including one up to Mont Ventoux.
Merckx came back to Tour de France after a gap year in 1973, when he was the first cyclist to manage the Vuelta-Giro double. During the previous years, between 1969 and 1972, Merckx won all the four editions he entered. His fifth overall victory in 1974 made him equal to Jacques Anquetil.
He won 3 flat and 3 mountain stages, also two individual time trials.
Freddy Maertens (1976)
Freddy Maertens' 8 stages victories in 1976 is really remarkable considering that the program of this edition suited rather to climbers like Lucien Van Impe (who won the race).
Nevertheless, Maertens won 3 individual time trials, 4 flat sages and a hilly one.