What is your symbolic understanding of the number five? Does it have significance beyond the historical origin of the first five?
Number five is significantly used in Sikhism. God created the body of five elements, there are five khands (level of creation) etc. The reason for choosing five Sikhs is best known to God Himself. But I think it ensures that the community can reach a sound decision without any disputes. One makes it a dictatorship, two can have an unresolved dispute, three leaves the possibility of two taking sides and overpower the minority one who might be truthful, four can have unresolved dispute when there are two on each side. So a minimum of five people are required to take sound and undisputed decisions.