In Persian, nouns are not pluralized after a number because the number itself indicates the quantity:

یک کتاب
yek ketâb
one book
دو کتاب
do ketâb
two books
چند کتاب
chand ketâb
a few books (any number betwen 3 to 9)

But:

همه‌ی کتابها
hame-ye ketâbhâ
all books (no particular number)

More examples:

دو برادر دارم
do barâdar dâram
I have two brothers
برای چند شب
barâ-ye chand shab?
For how many nights?