As far as actual signed models, not counting self proclaimed fashion models such as the girls from Omaha who is only 5’2″ but puts fashion modeling on her modelmayhem.com profile, the number of fashion models is about 3600 in the US. (By the way, the number of “active” models on Model Mayhem are 112,000 internationally.) The US Department of Labor puts models at around 4800 but not all of those qualify to fit into the fashion category of modeling.
I apologize that I will never be able to even guess what that number is in the entire world. As there is not enough data gathered as well as Mariana Leung-Weinstein stated, It depends on how one would classify what a fashion model is. What a fashion model is to someone in the Philippines would not be classified as such here in the US.