In Turkish, a plural declension is not needed if the noun is modified by a numeral.
(If it is used, it is emphatic.)
In fact, it is not needed with any modifier that already implies plural.
bir arkadaş (one friend)
iki arkadaş (two friends)
birçok arkadaş (many friends, a lot of friends)
çok fazla arkadaş (too many friends)
sayıda arkadaş (numerous friends)
Note all the situations in which -lar is not required.
The same is true of languages from other language families like Persian, Armenian, Georgian and Kurdish that existed in the region before Turkish, and of Turkic languages generally.