These are the countries with the most Jehovah's Witnesses (based off the number of people that attended the most recent Memorial Festival) according to Wikipedia as of 11/21/2020 A.D.
Note that although the people attending the yearly memorial provides interesting statistics, this does not constitute the number of Jehovah's Witnesses in a country. The number of people that are present also includes individuals that study the bible with them and potentially plan to become one of Jehovah's Witnesses. For accurate statistics, check the latest service year report at jw.org