All of them.
The fields are each equipped with three fixed cameras. One from behind home plate, the other from center field and the third in one of the dugouts to give a side view of the pitcher.
After practice, the D-backs coaches are able to cue up any pitcher and watch those they may have missed live or want to look at further. The video is also available for pitchers if they want to review their performance.
"We always try to tape as much as we can," D-backs manager Kirk Gibson said. "We store and put it in our system so you can go back and watch the whole session if you want to. We'll watch the tape more and see where they were trying to locate and how much they located."
Steve Gilbert is a reporter for MLB.com. Read his blog, Inside the D-backs, and follow him on Twitter @SteveGilbertMLB. This story was not subject to the approval of Major League Baseball or its clubs.