I'm mainly talking about 4-star-hotels in Germany. There are so many towels (see this question) that I usually would not use all of them. However, I was wondering if at the end of the stay, housekeeping usually replaces even those that appear untouched. If that is the case, I might as well be more lavish with my usage.
Yes, all towels are replaced when a customer leaves the hotel even if they look untouched. This is a common procedure in hotels. You can find some hotels where the management is trying to save money and will eventually not replace all but by default, in the vast majority of hotels, they are replaced.
Source : some housekeeping executives I have been discussing with about this and other topics.