hard to answer, for once, like in previous contests, many people wait till the last day to submit their entries, which makes it hard to figure out how many will there be, others could be waiting to the 2.6 beta, (I'm among those to finish my current entry, althou if it doesn't come in time I could work something out). The theme could be opened, I thought, anyway, that other themes "were posible" but you got "brownie points" for sticking to theme, I wouldn't mind a more open theme, but I have to say that it has been great fun creating a scene with a theme in mind.
To somehow measure how many people are going to try (which is a different thing than deliver) could answer in the contest thread stating the intend to participate and their general theme/idea, then they could go back to that post to edit and add the images.
I've always thought that the more we were compiting the better, mainly cause we learn a lot from each other (or at least I do) and also it usually pushes you to improve your own work, I could be days watching Enrico's renders and wouldn't get bored (and many others as talented as him) and they always make me want to learn more. So if it needs to be extended I wouldn't mind (much
Just my 2 cents