About 5,000 environmental groups participated in Earth Day 2000.
The figure comes from descriptions of the first Earth Day of the new millennium, which combined a global-warming focus with a clean-energy campaign. Participation extended well beyond a single national organization: environmental groups used online communication to coordinate activism, circulate information, and connect local events across borders.
Earth Day 2000 was reported as an international effort involving 184 countries. The approximately 5,000 participating groups therefore represented a broad network of environmental organizations rather than 5,000 individual events or 5,000 countries. The number is conventionally presented as an estimate, which is why “about” is important in the question.
A common error is to substitute figures from other Earth Day anniversaries. Earth Day 1990 mobilized an estimated 200 million people in 141 countries, while Earth Day 2010 involved an estimated one billion people and about 20,000 partners. Those figures belong to different campaigns. For 2000, the canonical organizational participation figure is 5,000 environmental groups, not 500, 15,000, or 50,000.