This is the actual number of ADVENT legions stationed in a region. Higher strength means tougher enemies and more reinforcements on missions.
Here is your field manual for not just surviving, but prevailing. xcom 2 long war guide
Gone are the days of taking a full squad on every mission. is the new key. You must assign soldiers to a mission, and they will spend time infiltrating before you can launch it. This is the actual number of ADVENT legions