We have it on our site and wonder how many people might not be able to use it.

I assume you mean Javascript and how many people have javascript disabled (Java is very different): Yahoo did some work on this a little while ago and reckoned on about 1% of their visitors -- but they do get 300 million+ per month to their home page. There are some studies that reckon the proportion is increasing as intelligent users put the shields up. You could run an analysis of your server logs or there are several simple techniques to test and analyse. Better questions are why are we using Javascript, what for and what are the other issues asscoiated (eg js navigation causing search spiders to ignore you).