I don't understand the question.
Is this about the number of bath towels used per person, from one washday to the next?
(my answer is 1 or 2. Most English people don't have showers everyday. And there's nothing wrong with letting one towel dry out and using it again.)
Or is it about the number of bath towels used per session?
(my answer is 1. I use a smaller (hand) towel for drying my hair.)
Or is it about the number you like to have in total so that you can rotate through your collection each washday?
(my answer here is 4.)