The past few years the board has selected 20-25 applicants. I have heard the number will be lower this year (15-20) so we will see if that holds true.
I have not heard of any specific field or specialty they are looking for, but I do know of the half dozen or so in my region that got picked up last board, a few of us were the only non-priors in some branch of the military so prior service helps a lot. Masters degrees are still vital as well as actual real world leadership/managment experience.
Best of luck to those that applied for the DCO board that met last week. Last year the results came out fast so I would not be shocked if those that got selected find out by the end of this week.