We can attempt to derive the number of members of the Order of the Phoenix and the number of Death Eaters from a couple of passages in the books.
In Order of the Phoenix Chapter 9 Moody shows Harry a photograph of the original Order and names the following people:
- Himself
- Albus Dumbledore
- Dedalus Diggle
- Marlene McKinnon
- Frank Longbottom
- Alice Longbottom
- Emmeline Vance
- Remus Lupin
- Benjy Fenwick
- Edgar Bones
- Sturgis Podmore
- Caradoc Dearbon
- Rubeus Hagrid
- Elphias Doge
- Gideon Prewett
- Fabian Prewett
- Aberforth Dumbledore
- Dorcas Meadowes
- Sirius Black
- Lilly Potter
- Peter Pettigrew
- James Potter
While it is possible there were other members not in the photograph, it seems likely that they tried to get all the members present for the photograph as Moody tells Harry that that was the only time he met Aberforth.
A number in the low twenties is also consistent with what we know about the numbers of the reconstituted Order, from Order of the Phoenix Chapter 4 (my emphasis):
“Is anyone going to bother telling me what the Order of the Phoenix —
?”
“It’s a secret society,” said Hermione quickly. “Dumbledore’s in
charge, he founded it. It’s the people who fought against
You-Know-Who last time.”
“Who’s in it?” said Harry, coming to a halt with his hands in his
pockets.
“Quite a few people — ”
“ — we’ve met about twenty of them,” said Ron, “but we think there
are more...”
If we take Ron's word for it, there are somewhat more than 20 members in the new Order (some of the old members are dead, and some new members were not around the first time). This is more or less confirmed by Mr. Weasley in the beginning of Deathly Hallows:
"And as there are around twenty of us, that greatly dilutes the power
of the Fidelius Charm.
We can derive the number of Death Eaters from something Lupin says in Order of the Phoenix Chapter 9 (my emphasis):
“Oh, Molly, come on, it’s about time you got used to hearing it —
look, I can’t promise no one’s going to get hurt, nobody can promise
that, but we’re much better off than we were last time, you weren’t
in the Order then, you don’t understand, last time we were
outnumbered twenty to one by the Death Eaters and they were picking
us off one by one...”
Thus, if there were 20 Death Eaters for each person in the photograph there would have been 440 Death Eaters in the original conflict. While it is possible that Lupin was exaggerating somewhat, there must at least have been many, many, many more Death Eaters than Order members.
When Voldemort returns to power, there are perhaps slightly more than 30 Death Eaters in the graveyard, if Harry is to be trusted:
All he had learned there was the Disarming Spell, “Expelliarmus” ...
and what use would it be to deprive Voldemort of his wand, even if
he could, when he was surrounded by Death Eaters, outnumbered by at
least thirty to one?
There were at least eight empty spaces there that Voldemort addresses:
Voldemort moved on, and stopped, staring at the space — large enough
for two people — that separated Malfoy and the next man.
He had reached the largest gap of all, and he stood surveying it with
his blank, red eyes, as though he could see people standing there.
“And here we have six missing Death Eaters . . . three dead in my
service. One, too cowardly to return ... he will pay. One, who I
believe has left me forever ... he will be killed, of course ... and
one, who remains my most faithful servant, and who has already
reentered my service.”
This knocks us up to about 40 Death Eaters. Note that as this is immediately after Voldemort's rebirth, everyone present must have already been a Death Eater the first time around.
Additionally, in Order of the Phoenix there is a mass breakout from Azkaban in which 10 Death Eaters escape. This adds on another eight Death Eaters (the Lestranges were already accounted for in the two-person gap that Voldemort addressed) thus bringing us to around 50.
Furthermore, it is unlikely that these 50 are the only Death Eaters. At most these are the only ones who have a Dark Mark. But we know from Deathly Hallows that only Voldemort's inner circle merited the Dark Mark:
Harry thought he knew why Greyback was not calling Voldemort. The
werewolf might be allowed to wear Death Eater robes when they wanted
to use him, but only Voldemort’s inner circle were branded with the
Dark Mark: Greyback had not been granted this highest honor.
Thus, it is possible that there were in fact many more Death Eaters, perhaps even approaching Lupin's estimate.
We also have the following comment from Sirius in Order of the Phoenix telling us that Voldemort had huge numbers on his side (my emphasis):
“Well, firstly, he wants to build up his army again,” said Sirius.
“In the old days he had huge numbers at his command; witches and
wizards he’d bullied or bewitched into following him, his faithful
Death Eaters, a great variety of Dark creatures. You heard him
planning to recruit the giants; well, they’ll be just one group he’s
after. He’s certainly not going to try and take on the Ministry of
Magic with only a dozen Death Eaters.”
This would fit with the above – a core group of Death Eaters, with several hundred other fighters not part of the inner circle (and Lupin's number may have been including this broader group).
The anti-Voldemort side probably also had additional people besides for the Order of the Phoenix, e.g. some of the Ministry, Mrs. Figg (who may be technically part of the Order but probably didn't do much fighting) and perhaps others, but those numbers are harder to account for.
