Most of the soi's have the main road or district as well as their number. The number is not unique but adding the road or district makes it unique. Your will find the next soi is Soi 3 Hua Na, then Soi 5 Hua Na and so on. The name distinguishes the number from other sois with the same number.
A good example would be Sukhumvit Road in Bangkok. All the sois have numbers as everywhere else but you would refer to each soi as 'Sukhumvit Soi 13, Sukhumvit Soi 15 etc or on the other side Sukhumvit Soi 22, Sukhumvit Soi 24 etc'.