I've got a pretty big world, that has grown into a lot of races.
It's pretty "humanocentric" in that humans used to be the only intelligent species on the planet, and still account for the most common species. They arose through plane old evolution, but things have changed in the years since the Great Fall, and now magic has changed the way things progress.
The wide range of human cultures means their relations with other races ranges from outright hatred and "racism" to intermingling. I have about a dozen human subraces described scattered around the setting.
The elves are actually another human subrace in my setting, just one further removed from the others. My setting is Earth, after a failed experiment into dimensional travel caused a pretty big screw up that resulted in several different timelines and and alternate realities being juxtaposed onto the earth.
The Elves are a displaced group of humanity from some point in the future who were found in a sorry state by a tribe of northmen. Their odd appearance and long lifespans were the result of cosmetic gene therapy, and when the northmen spotted them they assumed the strange folks were alfar. The "elves" were smart enough to run with that label, and ended up exploiting the humans long enough to get their shit together in the rough, medieval world they found themselves in. They present one face to outsiders, 'roleplaying' mysterious magical creatures, but have a different relationship with each other. The two great houses operate more like corporations than noble families.
Halflings are found pretty much wherever humans are, and I have specifically not explained why. They're helpful, jovial folks who live symbiotically with humans, but usually maintain their own enclaves within the larger cities.
The Dwarves are only found in the north, and are a species in decline. Like the halflings, their origins are mysterious, but they are much more rare. There is a sense that they are also a species displaced, like the elves, but they fit in among the northern society better than the elves.