Human beings are a single species.
A species is an incredibly broad basket.
Technically, if elves can have half-human children, they are the same species as a human.
The discussion about humans being a "single <something>" is usually about "race", of which not all humans are the same.
Well noooo!
Well Humans are one race... this is backed up scientifically and MOST settings don't really alter this (Eldar Scroll suggests humans are multiple species that are entirely different. Ignore the unfortunate implications!). This ALSO includes that humans are all the same species.
Other fantasy races are often multiple and are genetically divergent. "High Elves" and "Drow" though both being Elves are separate.
A Human + Elf Hybrid... would be a different species... a Cross Species (Though I guess it might count as both).
Unless in typical fantasy setting a cross between two races creates an entirely new race unlinked! DND Tieflings are this.
