Complex sentence: There are over 420 White Castle outlets, all in the United States and specifically in Midwestern states, Kentucky, and Tennessee except for a significant discontinuous smattering of outlets in the New York metropolitan region and one location in Nevada.
Split into simpler sentences: There are over 420 White Castle outlets, all in the United States and predominantly in the Midwest, Kentucky, and Tennessee. The exception is a significant smattering of outlets in the New York metropolitan region, and one location in Nevada.