Depending on your location, you can get dozens of channels over the air with a modest TV antenna. I have a fairly large antenna hanging inside my garage, and even being inside I can still pick up about 45 channels (main and sub-channels) in the Seattle area. Some days I even get a station near the Canadian border.
But you have to remember that OTA TV is either VHF (54-88 MHz and 174-216 MHz) or UHF (470-692 MHz). These are line-of-sight signals, and if there is anything in the way (terrain, buildings) it may not be possible to get a solid signal. With digital TV (which is all we have in the US) the signal has to be a certain level or you get nothing. With old analog TV, you might get a weak signal with wavy lines or other things that make the picture bad, but you could still get something. With digital, it's all or nothing. Go to this site
and enter your home address, and you will find out what potential TV stations you can receive. It will tell you the direction and distance you are from each transmitter, and give some basic advice on the size antenna you might need to receive them. Just remember that it's guide, and not a guarantee that you can receive them.