There is an article over at Fire Aviation about the number of Unable to Fill (UTF) requests for air tankers last year. Here is an excerpt:

New data that the National Interagency Fire Center released about the 2012 wildfire season reveals that almost half, or 48 percent, of the requests for large air tankers could not be filled. Of the 914 requests, 438 were rejected as “unable to fill” (UTF), meaning no air tankers were available to respond to the fire; 67 were cancelled for various reasons.