There are a few possible explanations. The word five
was sometimes used to express the concept a few
(see Leviticus 26:8 and 1 Corinthians 14:19). Egypt had many cities, and the word five
could indicate that only a few of them would worship God.
Other commentators believe that it is used simply as a round number to refer to a certain number of cities.
Some sources mention five specific cities: Leontopolis, Heliopolis, Migdol, Daphne, and Memphis.
The last possibility is that five is seen as half of ten, with ten being the number of completeness.
18 In that day there will be five cities in the land of Egypt that speak the language of Canaan and swear allegiance to the LORD of hosts. One of these will be called the City of Destruction.