The American Cleaning Institute recommends replacing your bath towel after using it three to five times, as residual moisture on damp towels can foster bacteria and other germs.
You can wash your towel less often if you only use it to dry off clean skin. If you have a cut, sweat a lot, or use it for your hands or remove makeup, wash it more frequently.
In humid climates, towels tend to stay damp for more extended periods, creating an environment conducive to bacterial growth, and therefore may require more frequent washing.
It's also better to keep a fixed schedule for washing towels. For example, washing them every three or four days, regardless of how many times you've used them.