Beyond personal experience, the closest thing you'll get to an objective guess is the TIOBE index, which currently ranks LISP/Scheme 21st with 0.470% market share. TIOBE uses search engine results to create the list. Obviously, you'll want to take the list (and any other guess at Scheme usage) with a grain of salt.
Shameless promotion: check out PLT Scheme. They've created a nice community, offer their IDE for many platforms, and give you a nice library selection out-of-the-box.
Here's an article describing PLT Scheme in a commercial app: http://www.untyped.com/downloads/icfp068-welsh.pdf.