One sacrifice is sufficient on behalf of all of you, because it is narrated in the Sunnah that one sacrifice is sufficient on behalf of a man and the members of his household.
Al-Tirmidhi (1505) narrated that ‘Ata’ ibn Yasaar said: I asked Abu Ayyoob: How was the sacrifice done at the time of the Messenger of Allaah (peace and blessings of Allaah be upon him)? He said: A man would offer a sheep on behalf of himself and the members of his family, and they would eat some and feed others with some.” Classed as saheeh by al-Albaani in Saheeh al-Tirmidhi.
The Standing Committee was asked:
If my wife lives in the same house as my father, is it sufficient for me to offer one sacrifice on Eid al-Adha on behalf of myself and my family, or not?
If the situation is as you describe, with a father and son living in the same house, it is sufficient to offer one sacrifice on behalf of yourself, your father, your wife, your father’s wife and all the members of your household, in accordance with the Sunnah.” End quote.
Fataawa al-Lajnah al-Daa’imah, 11/404.