I do not know how many came with the games originally but for my 1955, 1963, and 1971 Williams P&B’s I have 5 balls in each. You could probably get away with one but would have to wait longer between pitches as the previous pitched ball rolls down the ramp or playfield to be loaded again in the pitching mech. I like to run through games fast so at times it looks like multi-ball with a few balls still rolling back down the playfield that I didn’t get enough “wood” on