Although there is no restriction on number of characters in App Name. But if app name has more then 12 characters then it will truncate.
But
It's very difficult to find, how long iPhone and iPad App name could be without shrinking ?
Apple(iOS) shrinks the App name based on total width of the given name.
For example, width of char "i" is less than char "W". So if App name contains thin characters like "i", App name could have longer without shrink and if App name contains thick characters like "W", App name could have smaller without shrink.
FYI! iPad is having wider space than iPhone, so iPad App name could have more characters than iPhone.