Coach,
I've seen one mlb tryout and they threw somewhere between 14 and 20 pitches. The scout team tryout I attended was very similar with the difference being that there were quite a few college coaches at the scout team tryout. I wasn't close up at the mlb tryout so I don't know how they judged anything. The scout team tryout tended to follow velocity, with lefties having an advantage of course, but the pitchers were all pretty similar as to their control with only minor exceptions and the one pitcher who came in with a "rep" for pitchability and demonstrated good control made the better scout team despite topping out at 86 and having been there for a couple years. Reputations/returners, grade level and bloodlines had an impact but overall I didn't see any real issues with which kids ended up on which team or not on a team.