It can depend on the cat as to whether they'll get lonely if left alone and for how long you leave the cat alone for.
I had a cat that would do just fine being left alone and then another one would go crazy being left alone and needed a companion cat to keep it company.
If you plan to be away from your cat longer than a day then I recommend at least getting another cat to keep your cat company or having the cat looked after at a pet place.
If you're just gone for 8 hours per day to a job then leaving a radio on or getting your cat another cat to keep it company will work just fine.
Most cats do get lonely though if left alone so it's a good idea to use another cat or white noise to help keep the cat from going crazy and stay calm while you're away.