@Vee274 The most common allocation I see is two towels and two washcloths per guest per one week stay, but there are other variables. Some hosts give guests access to backup storage of extra towels and linens, and hosts who provide laundry facilities tend to assume that longer-staying guests will wash their own towels as needed.
If you offer a water feature such as pool or hot tub, or there's a beach nearby, a separate basket of outdoor towels would be very good to have.