There's no reason why headsets should be handled any differently than other bearings when it comes to how many balls to use. (the entire assembly is the bearing, consisting of inner and outer races aka cups and cones, the balls are just that - balls) . The object is to fill the outer race to capacity, while making sure that the balls aren't forced against each other.

Generally when filled, there will be a space of about half a ball. Since it's harmless to be 1 ball short, and totally destructive to have one ball extra, folks who are unsure may remove one.

One way to be sure is to line the cup with grease, pack in the balls, and place it against the cone and spin them together a second or two then pull them apart and look. If the balls are in a nice neat circle with grease swirls from the movement, all is well. If you see one ball up or to the side, you overfilled it. You'll also know because with an extra ball the two parts won't want to lay flat against and you feel some wobbling as you rotate them.