كَمْ لَبِثْتُمْ
How long did you remain? (al-Kahf 18:19)
Interrogative kam asks about number or quantity. Its answer specifies an amount, such as three days or ten books.
كَمْ كِتَابًا قَرَأْتَ؟
How many books did you read?
Kam is an interrogative noun, and “kitāban” is its accusative specification (tamyīz), identifying what is counted.
Do not confuse interrogative kam with declarative kam: “kam kitābin qaraʾtu!” may express great quantity, whereas the interrogative sentence genuinely requests a number.