It seems pretty obvious at the beginning: 3 girls and 5 boys. (Hermione, Lavender, Parvati, Harry, Ron, Neville, Dean, Seamus). But then in chapter 7 of Prisoner of Azkaban:
"R - r - riddikulus!" squeaked Neville. There was a noise like a whip crack. Snape stumbled; he was wearing a long, lace-trimmed dress and a towering hat topped with a moth-eaten vulture, and he was swinging a huge crimson handbag. There was a roar of laughter; the Boggart paused, confused, and Professor Lupin shouted, "Parvati! Forward!" Parvati walked forward, her face set. Snape rounded on her. There was another crack, and where he had stood was a bloodstained, bandaged mummy; its sightless face was turned to Parvati and it began to walk toward her very slowly, dragging its feet, its stiff arms rising -- "Riddikulus!" cried Parvati. A bandage unraveled at the mummy's feet; it became entangled, fell face forward, and its head rolled off. "Seamus!" roared Professor Lupin. Seamus darted past Parvati. Crack! Where the mummy had been was a woman with floorlength black hair and a skeletal, green-tinged face - a banshee. She opened her mouth wide and an unearthly sound filled the room, a long, wailing shriek that made the hair on Harry's head stand on end - "Riddikulus!" shouted Seamus. The banshee made a rasping noise and clutched her throat; her voice was gone. Crack! The banshee turned into a rat, which chased its tail in a circle, then - crack!- became a rattlesnake, which slithered and writhed before - crack! - becoming a single, bloody eyeball. "It's confused!" shouted Lupin. "We're getting there! Dean!" Dean hurried forward. Crack! The eyeball became a severed hand, which flipped over and began to creep along the floor like a crab. "Riddikulus!" yelled Dean. There was a snap, and the hand was trapped in a mousetrap. "Excellent! Ron, you next!" Ron leapt forward. Crack! Quite a few people screamed. A giant spider, six feet tall and covered in hair, was advancing on Ron, clicking its pincers menacingly. For a moment, Harry thought Ron had frozen. Then - "Riddikulus!" bellowed Ron, and the spider's legs vanished;
If you count, there are 8 forms the boggart takes. Since Harry and Hermione didn't do, that brings it to 10.
"But I didn't do anything," said Harry. "You and Hermione answered my questions correctly at the start of the class, Harry," Lupin said lightly.
Where did those 2 students go? Who are they? Why didn't JKR tell us about them? Presumably they are girls, because there are only 5 beds in the boys' dormitory.
five four-posters hung with deep red, velvet curtains.