How many students are in Harry's year in Gryffindor?

Question

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.

Answer 1

At first, I thought this was just a case of JK Rowling being bad at math, and forgetting how many students were in Harry's class. But I think the key is this part:

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!"

The boggart's other transformations explicitly occur when a student steps in front of it, but here there is no mention of anyone doing that. It seems to be transforming at random, without an external stimulus. This fits with Lupin's statement that it's "confused" - it doesn't know what form to take in order to elicit the fear it feeds on.

I thought it was explicitly stated, either in the book or the film, that the boggart got confused about what form to take, but I can't find any reference to it. Nonetheless, I believe this is the case: there were only 8 students in the class, and the boggart's transformations were the result of its confusion, not the result of some unmentioned student stepping in front of it.

Answer 2

As mentioned in this answer, there are apparently two unnamed Gryffindor girls in Hermione's year, in addition to Parvati Patil and Lavender Brown. JK Rowling has been asked about them in multiple interviews, despite them never having been mentioned explicitly in the canon:

Adele: Who are the two "unknown Gryffindor girls" in Harry's year?
JK Rowling replies -> Oh, I've just understood what you mean. I haven't got the notebook to hand and I can't remember! That's terrible. I'll try and remember before the end of the chat!
_{-- JK Rowling's World Book Day Chat, 4 March 2004}

MA: Yeah. Have you discovered the two missing Gryffindor students?
JKR: [Covers eyes] Ohh! [Frustrated.] I was going to go and get that for you, I'm sorry I haven't got it, I'll put it on my site.
_{-- The Leaky Cauldron and Mugglenet interview Joanne Kathleen Rowling: Part Three, 16 July 2005}

MA: Speaking of Gryffindor- students, do you have the name of those two Gryffindor girls?
[...]
JKR: Do you know what? I swear to you, I will find the damn notebook, (ma laughs) and I will put it on my website. I will put it on my website, (SU: Woo hoo!) that will be my gift to you.
[...]
MA: It’s just so funny because this thing has been- you know, it’s been, like, five years or something, just that question.
JKR: [...] I never mentioned them once in the canon, so…
_{-- PotterCast 131 J.K. Rowling Interview Transcript, 2 Jan 2008}

Answer 3

I don't know about in the books, but in the movies, the Slytherins were also in that DADA session. Although I realize that you are probably looking for an answer from the books, the other forms the boggart takes could be the fears of some of the Slytherins there as well.

Answer 4

As I mentioned in this answer, in Chapter Twelve of Order of the Phoenix we are explicitly told how many students are in Harry's Defense Against the Dark Arts class:

"It was murder," said Harry. He could feel himself shaking. He had hardly talked to anyone about this, least of all thirty eagerly listening classmates. "Voldemort killed him, and you know it."

Thirty classmates plus Harry equals 31 students in the class. (Of course, "thirty classmates" could just be an approximation.) That is certainly enough to cover eight boggart forms even without Harry and Hermione participating.