Can you trust the main page? Did you count the members yourself? Hmm? Hmm?
Ha! That is funny. I think that number is trusted.

But how many of them have entered? How many people have commented on these threads? That would give us a good idea.
Now that part we don't know. But by the hard numbers from sales and total members. We can make an educated guess of at least 8,000 members have signed up. That is a lot of competition.
http://www.gamesas.com/topic/1381804-dawnguard-beta-17/page__view__findpost__p__20943660
EDIT : That is if only 10% of the 80,000 360 users do it.