question about using des with number

Question

why are we using des with six ( chacun des six chateaux)  as six is the defined number

Maarten Kamp · Accepted Answer

Archana, 
this is effectively the same structure as the equivalent expression in English.  
It refers to each of a specific ‘ six chateaux ‘. 
In English this would require “ (each of ) (the six châteaux ) “. 
In French it is ‘ ( chacun de ) (les six châteaux) ‘  which with contraction becomes “ chacun des six châteaux “. 
The lesson covers ‘ chacun des ‘ with plural nouns, and this example is just a case of a specific number of things in the group.
https://www.wordreference.com/fren/chacun