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
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):