How many Jehovah's Witnesses are there?

Wikipedia claims that ~8 million Jehovah's Witnesses are engaged in evangelism, and their annual memorial of Christ's death has ~20 million attendees. But it doesn't exactly answer the question of how many members that Church has. My uninformed guess is that if there are 20 million attendees to the celebration, there could be up to 40-60 million witnesses (under the assumption that many of them not really actively practicing their faith). So, how many Jehovah's Witnesses are there? Is there any official statistic? Who counts as a "member"?