Welcome to New Leaf!
If you have fewer than 9 villagers currently living in your town, random villagers will move in. I have found that the random move-ins often prioritize the personalities that you are missing if you don't have all 8.
In addition to random move-ins, you can invite villagers from other people's towns, via Amiibo, or from the campsite. The campsite is a PWP you can build, but it is permanent so make sure you know exactly where you want it before you build it.
You can have 10 villagers, but the 10th villager will never be a random move in. You must invite them via one of the methods I listed above. Or you can pick up voided villagers from interacting with other people online even if you haven't invited them.
Villagers can place their houses anywhere they find room, so they can end up in very inconvenient places. But with the Welcome Amiibo update, they won't move on top of patterns as long as there's another place that they can move. So you can block areas off using patterns or even force a villager to move in a specific spot by blocking off all areas except a 3x3 space for their house.