You can initially have up to 9 villagers. Constructing the campsite lets you have a total of 10 villagers.
You exchange villagers by visiting the person you want to exchange with and talking to their villager who is already packed up in boxes ready to leave their town. When you do this, you're given the option to have them come to your town if you have room (8 or less, 9 or less if you have the campsite) and vice versa. The villager you're trading must be in boxes too.
Then you wait or time travel to the next day to have them leave your town, which completes the exchange in full.
Also, this sub-forum is dedicated to giveaways, trades, auctions and cycling. If you have any other questions, feel free to private message me c: