Humans are the dominant species of the Sidhe empire. In the main, races tend to reproduce with each other, though assignations across species are not unknown, be it humans with fae, or dwarfs with werebeasts, or demons with celestials. Perhaps the most infamous of these pairings was Laufey and Fárbauti, whose union produced Loki the Trickster (you can read their tale in The Celestial Concordance).
The fae have much longer lifespans than humans, but reproduce and mature very slowly. Mankind is quite the opposite, and this fecund nature would be a source of some anxiety, as humanity could simply overwhelm them with numbers. However, there is moderation of humanity’s birth rate for all sorts of good reasons, beyond just keeping us in check.
Population density is far less than on our real-life world. ‘The solution to pollution is dilution.’ And war keeps the human population down across the Empire. Humans are the bulk of the Sidhe army, pouring forth through portals opened to conquer world after world in her name. We are legion, but we are also leashed.
Ultimately, what stops humans from begin able to stand eye-to-eye with the Sidhe is our short lifespans.
Yes, certain humans may be enspelled to a long life, but those magics have their costs and are not applied de facto across the population. The standard human lifespan in the Empire (if not killed in war or contest or accident) might average 200 years. The corresponding Sidhe life might average 8,000. Very few Sidhe die of old age …the Sidhe die because they battle …or destroy themselves, when the burden of time and loss becomes too much.