The International Confederation of Societies of Authors and Composers (CISAC) can provide the figures for the number of member societies per country and their members.
Typically a country will have a 'mechanical rights' and a 'performing rights' society collecting licence fees and distributing royalties. Some countries have joint societies or may be part of a multi-country joint venture.
When a music composer and or song writer becomes a member of such a society they enter into agreements about their musical works and other Interested Parties of those works, such as publishers. The interested parties of a musical work can be checked online using the ISWC-Net service of CISAC.
For example "A HARD DAY'S NIGHT" has an ISWC (International Standard Work Code) of T-010.147.507-3 and its Interest Parties are two creators: LENNON JOHN WINSTON and MCCARTNEY PAUL JAMES
What can't be stated is the number of composers or authors that are not member of such music societies, and so the quoted percentage cannot be verified.