Never seen them yell numbers in college and I've seen a bunch of games so that's news to me. How on earth would a college coach think a SS could hear the numbers being yelled from the 1st base dugout in the last inning of a tight game with 2000 people there yelling? Hence why they use their fingers.They normally yell them out in college as well. And use fingers. And both. And...
Every college is different.
If you want good examples go to www.nevermissasign.com. About 1/2 the coaches I know use this to produce their wristbands.