In Persian, nouns are not pluralized when the quantity is precise. For example, we say “three book” instead of “three books” because the cardinal number “three” indicates the quantity precisely.

یک شب
⟨yek šab⟩
one night
دو شب
⟨do šab⟩
two nights
هزار و یک شب
⟨hezâr o yek šab⟩
one thousand and one nights
دو برادر دارم
⟨do barâdar dâram⟩
I have two brothers
سه برگ کاغذ
⟨se barg kâǧaz⟩
three sheets of paper
(“barg” is a classifier here)

Quantifiers are words that express quantity. The quantifier چند (“čand”, a few) indicates a quantity between 3 to 9. Therefore, it requires singular form.

چند کتاب
⟨čand ketâb⟩
a few books
برای چند شب؟
⟨barâ-ye čand šab?⟩
For how many nights?

Quantifiers such as همه (“hame”, all) do not indicate the quantity precisely. Therefore, they require plural form.

همه‌ی کتابها
⟨hame-ye ketâbhâ⟩
All books
همه‌ی بچه‌ها
⟨hame-ye baččehâ⟩
All children