A gap, typically less than what one ball would fit is both normal, and necessary to the function of a ball bearing.

A properly built, and/or adjusted will have all balls in contact and under a slight load at all times. The overall bearing load from the weight on the axle is added to the load at the bottom, but unless the bearing is overloaded, doesn't change the other loads.

As such the force on the balls is purely radial, and happenstance and the lube's hydraulic pressure will cause the balls to spread themselves out somewhat equally. If the balls were overcrowded and made contact with each other it would prevent that natural spreading and cause friction as the adjacent ball surfaces rubbed past each other.

By the same token, a no gap design would make bearing construction nearly impossible, because ball to ball contact would prevent the radial contact necessary. By the same token, if the gap is too large the load might force the axle down slightly between the lowest balls, spreading them apart, and increasing both wear and vibration.

As for your 10th ball, odds are that it came from the other side, though on some hubs there's a recess behind the cup where an extra ball that was dropped during assembly could hide harmlessly until discovered by the first person to look.