The interrogative pronoun nan (= nani), when combined with a counter, is used when expressing a question about quantity: "How many ...?" The interrogative pronoun ikutsu ("how many") is used for asking the quantity of specific items such as apples and teacups, for which there is not specific counter. Such items are counted with native-Japanese numerals, or with Roman numerals pronounced in Japanese: hitotsu, futatsu, mittsu, etc.
Hon wa nan-satsu arimasu ka.
How many books are there?
Ryūgakusei wa nan-nin imasu ka.
How many foreign students are there?
Chūshajō ni kuruma ga nandai arimasu ka.
How many cars are there in the parking lot？
There are ten.
Suisō no naka ni sakana ga nanbiki imasu ka.
How many fish are there in the water tank?
Akai sakana ga ni-hiki to aoi sakana ga san-biki imasu.
There are two red ones, and three blue ones.
Kago no naka ni ringo ga ikutsu arimasu ka.
How many apples are there in the basket?
There are five.
Hope this helps!
°˖ ✧◝(○ ヮ ○)◜✧˖ ° こゆうきあいはら °˖ ✧◝(○ ヮ ○)◜✧˖ °