There is only a single speaker at the bottom of every iPhone ever made. When looking directly at the bottom of the device with the home button facing up, that single speaker is on the right. The grill on the left covers the telephone microphone.
See the pictures about mid-way down the tech specs sheet - iPhone X - Technical Specifications - Apple
Stereo on the iPhone 7, 8 and X is achieved using that single bottom speaker and the front earpiece speaker simultaneously.
As Michael says above it’s not actually a speaker, it houses the microphone and acts as an air vent for cooling and the altimeter while keeping the phones ip67 water resistance. The stereo speakers are situated in the front facing earpiece and the bottom right vent and work simultaneously.
Why left bottom speaker on iPhone X is not working?
