This states 10 balls/side.
VeloBase.com - Component: Shimano HB-A550 / FH-A550, RX100
I'd expect that count too, from that era.
You might look at-
Husky Bicycles Bicycle Hub Axle Shimano RM50 Front 140mm Bolt-On
I've had good service from them, but can't really attest to quality, since they are in my spare parts bin or gone on CL flippers.