Competitors per Judge is the total number of Competitors that a Judge will be assigned (asked to evaluate) when they check in. Assignments are made automatically when Judges check in, unless you've manually pre-assigned Competitors to a Judge. If you manually pre-assign Competitors to a Judge, their assignments won't change on check-in.
If a Judge is in a Group, he/she will be preferentially assigned Competitors from that Group.
Please note: This is affected by Wildcards. Wildcards are the number of Competitors assigned to a Judge regardless of Group. This is useful when there aren't enough Judges for one or more Groups