Can a front wheel have a different number of bearings on each side?
It's not unusual for an asymmetric shaft (on some rear wheels or solid-crank bottom brackets, eg) to have different numbers of balls in one side vs the other. But it's a little unusual for a standard front hub.  And if there's intentionally a difference then there would be a small but visible difference in the size of the races as well.

Likely the hub was serviced in the past and one ball was lost (or you lost one somehow).  But it's not really terribly serious.  It's often recommended (as a general rule for ball bearings) to have "a full race minus one" vs having the race completely full, and your typical 10-ball bearing can survive reasonably missing 2-3 balls (though that's not ideal).

However, since you have things apart it would be worthwhile to replace the balls.  (Don't mix old and new balls.)  See if ten fit comfortably.  If they fit, and there's still room for a fingernail between them, then use ten, otherwise go with nine.