its based on a lot of factors including the number of local stations broadcasting in you area, your distance from the towers, and how good your antenna is.
as far as antennas go, I have a few tips.
Some locations need large outdoor antennas to get reasonable reception.
There is no such thing as a 'digital' or 'hd' antenna. There are only UHF and VHF. Make sure you get one that does both with reasonable specs.
As with analog some channels come in better with different positions. Digital is a bit harder to position because its either receiving enough of the stream or it isn't. There are no static vague images.
AND You probably need a digital tuner if you TV doesn't support it.
Go here and plug in you zip code to see roughly what you should receive.
Here is a short description of TV frequencies.