Original Order of the Phoenix
Moody shows Harry a photo of the original Order of the Phoenix, which includes quite a few names (highlighted in bold):
“Original Order of the Phoenix,” growled Moody. “Found it last night when I was looking for my spare Invisibility Cloak, seeing as Podmore hasn’t had the manners to return my best one…. Thought people might like to see it.”
Harry took the photograph. A small crowd of people, some waving at him, others lifting their glasses, looked back up at him.
“There’s me,” said Moody unnecessarily, pointing at himself. The Moody in the picture was unmistakable, though his hair was slightly less gray and his nose was intact. “And there’s Dumbledore beside me, Dedalus Diggle on the other side… That’s Marlene McKinnon, she was killed two weeks after this was taken, they got her whole family. That’s Frank and Alice Longbottom—”
Harry’s stomach, already uncomfortable, clenched as he looked at Alice Longbottom; he knew her round, friendly face very well, even though he had never met her, because she was the image of her son, Neville.
“Poor devils,” growled Moody. “Better dead than what happened to them… and that’s Emmeline Vance, you’ve met her, and that there’s Lupin, obviously… Benjy Fenwick, he copped it too, we only ever found bits of him… shift aside there,” he added, poking the picture, and the little photographic people edged sideways, so that those who were partially obscured could move to the front.
“That’s Edgar Bones… brother of Amelia Bones, they got him and his family too, he was a great wizard… Sturgis Podmore, blimey, he looks young… Caradoc Dearborn, vanished six months after this, we never found his body… Hagrid, of course, looks exactly the same as ever… Elphias Doge, you’ve met him, I’d forgotten he used to wear that stupid hat… Gideon Prewett, it took five Death Eaters to kill him and his brother Fabian, they fought like heroes… budge along, budge along . . .”
The little people in the photograph jostled among themselves, and those hidden right at the back appeared at the forefront of the picture.
“That’s Dumbledore’s brother, Aberforth, only time I ever met him, strange bloke… That’s Dorcas Meadowes, Voldemort killed her personally… Sirius, when he still had short hair… and… there you go, thought that would interest you!”
— Order of the Phoenix, chapter 9 (The Woes of Mrs. Weasley)
I count 18 people in that photograph. There are a few people who aren’t listed, but we know or can assume were members of the original Order:
- Lily and James Potter
- Peter Pettigrew (seen in a photo in the film adaptation)
- Minerva McGonagall
There are also some individuals whose participation in the first war is unknown. They were all members of the reconstituted Order, but we don’t know either way if they were part of the original Order.
- Arthur and Molly Weasley
- Mundungus Fletcher
- Severus Snape. Although he turned spy before Voldemort’s downfall, it’s not clear if he was inducted into the Order before the end of the War.
- Arabella Figg. She was watching Harry since he moved to Privet Drive, so she was affiliated with Dumbledore since the First War, but we don’t know if she was part of the Order.
- Olympe Maxime. Definitely part of the second-order, as she accompanies Hagrid on his envoy to the giants, but no evidence either way about whether she was in the original Order.
- Hestia Jones. She was trusted to retrieve Harry in Order of the Phoenix, so was definitely part of the reconstituted Order.
So we can guess that there were maybe 25-or-so people in the First Order. There will be people not seen in the photo above or listed independently, but it won’t be a huge number.
Reconstituted Order of the Phoenix
We lose 8 people from the photograph due to death, and we never see Aberforth either. Lily and James were both killed, and Pettigrew defected.
The ambiguous people listed above get added to the total, which adds 7 to the total.
But of course, new people get recruited, including:
Kingsley Shacklebolt and Tonks. They definitely weren’t in the original Order:
“We’ve managed to convince a couple of people, though,” said Mr. Weasley. “Tonks here, for one — she’s too young to have been in the Order of the Phoenix last time, and having Aurors on our side is a huge advantage — Kingsley Shacklebolt’s been a real asset too. He’s in charge of the hunt for Sirius, so he’s been feeding the Ministry information that Sirius is in Tibet.”
— Order of the Phoenix, chapter 5 (The Order of the Phoenix)
Bill and Charlie Weasley, both of whom would have been too young for the First War.
The pool has thinned somewhat, and I’d guess that it’s more like 20-or-so at the start of the second war, but obviously some of these people get killed as the war continues.
Honourable mentions must also go to Harry, Ron, Hermione, Fred and George, and Ginny, who all fought alongside the Order in the second War, but I don’t know if they were ever formally inducted.
I do not claim this is a complete list of members – I’m sure I’ve missed a few – but I believe it is complete enough to get rough estimates for numbers.