There will probably be a lot of entries - not as many as you might think, though. I notice a few people here seem to think they can't come up with an entry in time...we've got two weeks to turn in one page, and they don't think they can do it? *Sigh*.
Bit of a tangent: yes, I know a lot of us will want to give SERIOUS thought to our entries. Still - don't work out everything now. Just come up with the major points. Figure out what you REALLY need to say and say it. To use an analogy from our hobby - if you were painting a miniature, would you start with the pupil of the left eye? Of course not! You'd paint the big stuff first, and if a color was going to be particularly prominent on a given figure, you'd do a full coat of it. You paint with broad strokes, you build up a few layers, and THEN you concentrate on the smaller details. And everything you'll do will be like that. Work on the general ideas now, and cross the other bridges when you come to them.
(While you're waiting for the judging, yes, you should give some extra thought to your entry, just in case, but for now - try to come up with the essentials.)
I should note that I'm really reluctant to give any advice here, but I take comfort in the fact that no one listens to me.
Anyway, a VAST number of entries will be thrown out on first sight - for a variety of reasons:
1. Failure to follow the guidelines.
2. Poor spelling/grammar.
3. Failure to follow the guidelines.
4. Too much "fudging" with the guidelines - WotC MIGHT, conceivably, accept an entry written in 10-point PC text. I believe, however, that it was AV who pointed out that they would toss entries written in 6-point text with no margins. "One page" does not necessarily mean "anything so long as you can fit it on a single page". (Incidentally, I'm not sure they would accept 10-point text - I plan to write my entries in 12-point to be on the safe side. These days I generally prefer 10-point, but there you are.)
5. Too similar to an existing setting. (WotC is probably not looking for a clone of Ravenloft or Spelljammer.)
6. Failure to follow the guidelines. No. REALLY.
WotC is going to get a LOT of crap - Sturgeon's Law, don'cha know. And most, if not all, of said crap will be recognized and thrown out straightaway.
I'd say maybe 1000 entries will actually make it to the judges. They will skim those, select a much smaller number to read, and bring it down to the final ten.
I wouldn't be surprised if there are up to 100,000 initial entries, however.
Edit: Just wanted to mention something - I'm submitting at least four entries. All have been floating around in my brain for a while, and I've given serious thought to all of them. I imagine many of you have done the same. Even so, the "broad brushstrokes" suggestion still applies - I hope it doesn't really need saying, but don't give WotC the tiniest of details JUST yet.