Their assistants lived in the villages outside the city, and every seven days a group of them would come into the city and take their turn at guard duty.
The Levites and the people of Judah followed Jehoiada's orders. The guards going off duty were not allowed to go home, and so each commander had all his guards available--those going off duty as well as those coming on duty.
Even though the Law of Moses says that the priests must be descendants of Levi, those priests cannot make anyone perfect. So there needs to be a priest like Melchizedek, rather than one from the priestly family of Aaron.