After the death of Solomon, Israel was divided into two parts, Israel in the north (10 tribes) and Judah in the south (2 tribes). From that time until the conquest of Judah by Babylon, there were 20 kings of Judah, not 8.
Here is a link which has a listing of the kings of Judah. http://www.jewishvirtuallibrary.org/jsource/History/Judah.html
I hope you have found this helpful.