What is the largest Rugby Union league in the world?

The Guinness Pro14 is the largest rugby union league in the world, featuring teams from the Celtic nations of Ireland, Scotland, Wales, and Italy. The league was formed in 2001 when the Celtic league merged with the Italian Super 10. Each season sees teams from the four nations compete for the title, with the top two teams entering the Champions Cup for the following season. The league also offers Rugby Europe International Championship, an annual international rugby union competition for European nations. In addition to the Pro14, the World Rugby Sevens Series is the largest rugby union tournament in the world, featuring 24 teams from six continents.

full article