OotP: Return of the Numbers Problem

[Blaise]

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Even though it's not a huge spoiler really... I've been doing some 
counting (and incidentally wondering whether LOON would take me on) 
and I noticed something. According to Lupin, back during Voldemort's 
first shot at world domination, the Order was outnumbered by Death 
Eaters 20 to 1. Now there must have been at least 20 members of the 
Order then, judging by the names and ages of the current members and 
the photo Moody shows Harry. So that means at least 400 Death Eaters, 
perhaps fewer allowing Lupin some leeway for exaggeration - maybe 
they were really only outnumbered 10 to 1. So my question is, where 
have they all gone? Only about 20 are mentioned in the books, there 
weren't anywhere near 400 at the gathering at the end of GoF, and 
Voldemort only mentions six missing. Presumably some are in Azkaban 
too, and some killed by Voldemort, like Regulus Black. But still, I 
can't believe there can be some 350 missing. Is this part of the 
eternal number-of-wizards problem? A Flint? Or does JKR have 
something up her sleeve? 

Blaise