The website monarchwatch.org offers – among many other things – data on monarch populations year by year.  Since the monarchs overwinter in only a few areas, winter populations can be measured fairly accurately.  The bad news is that 2014-15 winter was the second-lowest population total ever recorded for monarchs, the monarchs covering 1.13 hectares of forest.  The good news is that the size of the population was almost twice what it had been in 2013-14 (o.67 hectares).  After three consecutive years of precipitous declines, this is the first year populations have increased.  For full data take a look at their site.