it's rare to start again. This article is from 2014. Zach has 20 starts so he would be right at the top of the list for # of starts. If he doesn't start another game he will be the highest draft pick QB to start 20 games or less in the NFL. You can add the following names to the list who were drafted in the 1st round and have less than 20 starts (Even Jamarcus Russell had 25 starts):

Trey Lance
Jordan Love
Dwayne Haskins
Josh Rosen
Paxton Lynch
Johnny Manziel