The 10th villager comes from the campsite - you have to invite them in, if you don't like who you have in there, say no when they ask to move in.
Or -the villager can come from you visiting or being visited by another player (you can get the villager from their void).
Or - you can get via streetpass (again from a players void)
or - you can go to a town who has a villager in boxes and ask that villager to come to your town.
I don't think you get a 10th villager randomly generated by the game.
With 9 villagers you can have a villager move out before you get the 10th, if that happens, ie you go down to 8 - you can get a 9th game generated villager or via any of the above.
10 villagers is the max.
You are correct, on every guide there is it says it has to come from a campsite,streetpass,a friend that has a villager in boxes and you invite them to live in your town, or the void!