This is the Tagalog equivalent of the English interrogative pronoun how many. The answers to ILAN questions always include numbers or other words that express quantity (how much of something). Here are some examples:

Ilan ang kapatid mo?
Pito (ang kapatid ko).

Ilan ang estudyante ng Tagalog?
Labinlima (ang estudyante ng Tagalog).

Ilan ang klase mo tuwing Lunes?
Tatlo (ang klase ko tuwing Lunes).

Ilan ang dumating na bisita kagabi?
Marami(ang dumating na bisita kagabi).

Ilan ang batang naglalaro sa labas?
Iilan lang (ang batang naglalaro sa labas).

How many siblings do you have?
(I have) seven (siblings).

How many Tagalog students are there?
(There are) fifteen (Tagalog students).

How many classes do you have on Mondays?
(I have) three (classes on Mondays).

How many guests came last night?
Many (guests came last night).

How many kids are playing outside?
(There are) only a number (of kids playing outside).