Each set has a count of 2 uses before your Sim will need to do laundry to restart the count. This does not work with other cc Towels or EA towels yet. Each time they take a shower or bath, the towel usage will go down by one until you'll either need to buy another set or do laundry. When in-game, your Sims will need a Totally Necessary towel set.
In the 0.2.0.0 update, your laundry tasks are about to get a little longer. With this set used, your Sim will have to keep up with their clean towels on hand for each Sim.
This mod introduces mundane objects and activities into the Sims 4 that make Sim Life that much more realistic in Earthly terms.