@Pia17....Every guest is different, some will think it's like a hotel and a new set of towels will appear every day, others (in fact most) are happy with the same towels for 3 or 4 days.
We provide three complete sets, and virtually all overnights and two night stays will only use one set. For a week stay I will ask after 5 days how the towel supply is going and in 95% of cases they will say, fine!
You will assess as you go along but I would say start with a minimum of two sets and see where it goes from there!
If you want some degree of certainty you can explain in your house rules that for longer stays guest towels will be changed after 3 nights...If they are not happy with that, they will let you know.
Cheers.....Rob