This is really impossible to say... there are so many possible keywords and combos out there.
What can give you an idea is to look at tools that tell you the number of searches for a given term in a particular period.
this is the Overture search engine's tool. The number of searches in a month is only on the Overture engine though, so you need to refigure that to other engines.
I hear figures for Overture like 5 to 8% of all searches... and for Google anywhere from 40 - 60% of all searches, so you could extrapolate that whatever number of searches are done on Overture, approximately 8 times as many will be done on Google. There are an awful lot of factors that calc doesn't take into account (demographics, etc), but it might give you a ballpark figure.
Another popular tool is Wordtracker - they are a pay service, but they have a free trial. www.wordtracker.com.
That said, even if you identify the number of searches for a term, you still have to identify how many are going to click on your listing.
:) The fun never ends.