The new Ultimate Ears Premier IEM has a whopping 21 separate drivers on board. Note: this is an in-ear.
Separate from the 21 drivers in the Ultimate Ears Premier IEM, a five-way crossover filter is also built into the compact housing of the earbuds. Each driver is individually tuned to cover a specific part of the audio spectrum as accurately as possible. The frequency range runs from 5 Hz to 40 kHz. To bring out those very low frequencies, the manufacturer has built in two dual sub-bass drivers that work in parallel with the four dual mid-low drivers.
As you might expect, you won’t exactly get this technology for next to nothing; the Ultimate Ears Premier IEM costs $2999.