Down a row 20 people long you can get them all in focus by using a wide angle lens since they have a much wider DOF. I was talking to someone this morning. Lets assume they are using a DX camera with the Tokina 11-16
Set the lens at 11MM, F8 and focus at a distance of 3 feet. Shutter speed at least 1/30 or 1/60 just to make sure that people moving will not effect the shot. ISO to whatever it takes to get the exposure correct.
Everything from 1.37 feet to infinity will be in focus. This is called the hyperfocal distance.
This also works great when you are shooting in very dark so you can focus before you even go into the area
View attachment 228649
Online Depth of Field Calculator