I think, people don't like signs. In my opinion, the amount of towels you used to provide, is sufficient. But you may offer to hand out additional ones as need be. Usually, that has the effect that guests, if they have to ask for something, behave more considerate. - I learned my lesson. Usually, I just pointed out the linen closet in case they needed more than the initially provided ones. One of my newest, very pretty, beach towel was taken. They are not exactly cheap. So now, I will offer to provide beach towels at the time they are leaving to go to the beach, so they know, they get a certain amount and it will be easy for me to get the same amount back.