There are many federations and rankings in Muay Thai as well as in boxing, BJJ and other martial arts. Often, many fighters are quickly „World Champions“ although few fights were contested or the titles are only called so but are not international.
One of the leading associations here is the IFMA. The rankings are worldwide leading, and here there are strict rules who really becomes number 1 and who may fight for titles.
Check out the site – here you will find all the information: https://muaythai.sport/professional-rankings