This is a very simple concept, and I was quite surprised when I realized that English has no word for it (or even a simple way to phrase it). The word எத்தனாவது (roughly etthanaavadhu) in Tamil is hard to translate into English. The closest you could get, if you took some liberties with English is "which-th".
This is a "question word" like "what", "who", "where", "how" etc. Let me illustrate its usage with an example. Suppose A is B's son. The information you want to get from A is:
If A is B's n'th son, what is the value of n?
That isn't really English, of course, and there is really no direct and concise way of asking A a question in English that would elicit the information you want. You'd have to get this information in some roundabout way. Maybe you could ask: How many older brothers do you have, (who were also born to B?)? or How many sons did B have before you? and then add one to whatever answer you got.
If English had this "which-th" word, you could phrase the question thus:
Which-th son of B are you?