Estimates are rough, but it is believed that among all intelligent species there’s a total of ~3 billion people in the world. Although the many fantastical creatures, collectively known as “the Mythix”, are the majority, mankind is still by far the single most populous species, making up 1/3 of the total. The Merfolk come second, closely followed by Satyrs, Jotunns, Gnomes, Alkonost and Nokken, which collectively account for another billion people. Various other Mythix species make up the remaining 1/3.
Similarly to how various human ethnic groups can look quite differently (African, Asian, Caucasian etc.) but all still belong to the same kind, i.e. mankind, many Mythix who have been known with different names and descriptions among the various human cultures are actually part of the same race. For example, Selkies and Merrows are just some of the Merfolk, Fawns and Leshy are just different types of Satyrs, Orcs and Giants are also Jotunns, Gnome is an umbrella term for all the ”Little folk” which includes Dwarves too, Alkonost are often better known as Harpies and Garudas and the Nokken are also called Kelpies.
Additionally, in the wildlands exist “feral Mythix” like Gryphons and Jackalopes, who however aren’t considered akin to intelligent species, but rather closer to wild beasts (although it’s a debated and controversial topic for some).