There are no definites on how many people will enter your competition. We rarely, if ever, will ever tell someone that they'll get x number of entries simply because we think it's not possible to do this. However, we can say that a free prize draw will probably get more entries than a caption competition and that a big prize will be more attractive than a small one.
Another important point is don't assume that because there are thousands of people entering competitions online that this equates to instant traffic and entry volumes. A good generic audience prize with a well put together competition should be enough to get you a decent number of entries.
The key to "number of entries" is simply to use your common sense. We once had a Wedding Car Hire firm in Scotland complaining because no one was entering their "win a wedding car" competition.
The fact that it had locality and specialism associated with the prize instantly limited the amount of interest the competition had to a wide audience. Sadly, the siteowner failed to comprehend this!
The only way you'll know how many entries you'll receive and how well your competition performs is to run a couple of trials.
