I've had several 10th villagers via streetpass - ie they have come in via a stranger's town. Been lucky and got Whitney that way - she sings me her old town tune sometimes, told me she came from Sam town, found the players house in my HHA plaza.
That 10th empty space can be a problem - as the game tries to fill it - either from streetpass or from the void of a town you visit or are visited by.
So some players don't trade/visit and disable streetpass to prevent it happening - cos you have no control over who that 10th villager is.
You can invite a villager into that 10th spot - from your campsite (in town or via their RV) or via another town. (if they are in boxes in that other town).
So you could use an amiibo card or a compatible figurine (like some of the Zelda BoTW ones).
So I'm basically saying the same as Ably.Saucey