Headline seems to be misleading; the article itself and the primary source (https://makeamazonpay.com/) indicate that some form of protest will be going on in 20 countries; in not all of the countries is it a strike by Amazon employees.

I couldn't find an explicit list of the 20 countries, but the primary source has a map of where events will be taking place. By clicking each point on the map, I think I counted them all (I counted 21):

     1  Argentina
     2  Australia
     3  Austria
     4  Bangladesh
     5  Belgium
     6  Brazil
     7  Cambodia
     8  Canada
     9  England
    10  France
    11  Germany
    12  India
    13  Ireland
    14  Italy
    15  Netherlands
    16  New Zealand
    17  Poland
    18  Slovakia
    19  South Africa
    20  Spain
    21  USA