Complex sentence: If the supra is very small, in someone's home with only a few guests, the tamada won't be chosen, but the head of the house will simply assume the role of tamada.
Split into simpler sentences: If the supra is very small, in someone's home with only a few guests, the tamada will most likely be the man of the house. He won't be chosen, but will simply in his role as head of the house, assume the role of tamada.