Active Regions (AR) are either sunspots developing or fading. If they are prominent enough they will be given a number. An AR may not necessarily be visible as a "spot" in white light.
The NOAA (National Oceanic and Atmospheric Administration) are supposed to be guardians of the sunspot (aka Active Region) number. The webpage I most commonly go to is
http://www.swpc.noaa.gov/ftpmenu/forecasts/SRS.html which is part of the NOAA Space Weather Prediction Center
http://www.swpc.noaa.gov/alerts/index.html where they give the AR number, location and Z, or Zurich, classification for the sunspot group. I am not sure what visual parameters they use to trigger the assigning of an AR number as the imagers often see sunpsot groups way before NOAA and on some occasions they have not even given a number to an obvious sunspot group, its all a bit of a mystery!
Hope that helps
