The language used suggests that remedial lessons weren't an unheard of thing at Hogwarts, but there isn't much information about them, especially not enough to suggest how many students in each year may be taking them.
Snape says that nobody could deny Harry needs the extra lessons:
“I will expect you at six o’clock on Monday evening, Potter. My office. If anybody asks, you are taking remedial Potions. Nobody who has seen you in my classes could deny you need them.”
And Zacharias Smith didn't seem surprised at the concept when Harry mentioned he was taking remedial Potions:
“I’ll let you know in the usual way when the next one is,” Harry said over and over again, “but I can’t do it tonight, I’ve got to go to - er - remedial Potions.”
“You take remedial Potions!” asked Zacharias Smith superciliously, having cornered Harry in the Entrance Hall after lunch. “Good Lord, you must be terrible. Snape doesn’t usually give extra lessons, does he?”