You can have up to 16 villagers in your void.
The most recent voided villager has a chance of moving into someone's town if you wifi them/street pass them.
Clearing the void = You've recently voided a villager, and then you street pass somebody/wifi somebody. Your void is now clear. Your void can be cleared even if the person you visit has 10 villagers.
Voided villagers = Villagers that didn't get adopted
Getting voided villagers = The villagers that didn't get adopted go to the person's town you visited to with 9 or less villagers
A "voided villager" is just a villager that moved out of your town without being adopted by anyone. When a trader comes over for a trade, the voided villager might decide to move to the trader's town without warning. c: