The answer is, you’d better get a pair instead of one. Except for the situation that one of the ears is unable to wear a hearing aid like ear infection or other situations which make it impossible to wear a hearing aid.
Because most people have hearing loss in both ears, especially the seniors, who lose hearing for sake of age. With a pair of hearing aids, the user can keep both side of the hearing in the same level. The brain processes sound through what's known as binaural hearing, it normally does this by comparing the qualities of the sound signals that come through each ear—their relative loudness, their frequencies, and the time it takes them to travel through the ears. But the brain can't locate a sound as well if sound signals are always louder through one ear.