The Rulebook, page 10: "At Area contests, there must be an equal number of voting judges from each club in the Area, or a minimum of five voting judges. In addition to these voting judges, a contest chair, chief judge, tiebreaking judge, two counters, and two timers must be appointed."
Which means: If you choose to have internal judges, you need one voting judge from each club in your Area, regardless how many clubs you have.
Or, if you have external judges, you need a minimum of five voting judges who will not be competing in any of the four contests that they will be judging.
That means, if they have placed in that contest in another Area (or in the club and their Area conference has not yet happened), they shouldn't be a judge.
You'll also need a chief judge and a tiebreaker judge in addition to the above. It is best practice to have these two be external in either case.