Generally, I'd prefer either no subraces or a just a handful of subraces, if appropriate. I.E. in my Rhunaria homebrew campaign, the only playable race with subraces is elvenkind, and they only have 2 subraces besides the standard jungle elves of Rhunaria; dark elves and sea elves were both created from normal elves by the Mage-King centuries ago, near the end of his magical experiments (after making a lot of Rhunaria's aberrations, monstrous humanoids, and other wierd critters). The dark elves turned out as he wanted, amoral, elf-centric, and loyal to the Mage-King, with greater arcane affinities than normal elves, but the Mage-King was unsuccessful in changing the sea elves' minds through his experiments, so instead of conquering the seas for him as he had created them to do, they just fled and stayed in hiding until the Great Rebellion had toppled Linaerelesti and its Mage-King.
No other subraces are known in Rhunaria except for the different varieties of dragon and giant, and no one really knows why they have different varieties, though there are theories. The PCs recently learned from a gold dragon that dragonkind is descended from the divine dragon Xaritheon (whom no humanoid religion mentions nor recognizes). The gold dragon said that Xaritheon bred with ten different great beasts of the land, each bearing twin children, half- dragon and half-beast. Each pair mated and produced a clutch of eggs that were the first true (worldly, only semi-divine) dragons, possessing none of the mortal beast blood of their parents, only the divine dragon blood. This, according to the gold dragon, is what produced the five chromatic and five metallic dragons (as mortals tend to call them, anyway). Of course, the different Rhunarian religions have their own views on how this or that was created, and there's no telling who's right......