In the book, it was nine:
The white horse leaped forward. The hobbits ran down the slope. Glorfindel and Strider followed as rear-guard. They were only half way across the flat, when suddenly there was a noise of horses galloping. Out of the gate in the trees that they had just left rode a Black Rider. He reined his horse in, and halted, swaying in his saddle. Another followed him, and then another; then again two more.
One, and another, and another, and two more make five;
At once the white horse sprang away and sped like the wind along the last lap of the Road. At the same moment the black horses leaped down the hill in pursuit, and from the Riders came a terrible cry, such as Frodo had heard filling the woods with horror in the Eastfarthing far away. It was answered; and to the dismay of Frodo and his friends out from the trees and rocks away on the left four other Riders came flying. Two rode towards Frodo: two galloped madly towards the Ford to cut off his escape. They seemed to him to run like the wind and to grow swiftly larger and darker, as their courses converged with his.
And four more make nine.
And then there were none.