A short-term rental does not draw water like a family home. The draw comes in bursts. One group checks out, the laundry gets run, the cleaner runs the dishwasher, and the next group arrives and showers within the same afternoon. Sizing has to follow that pattern, which is why bedroom count alone tells you very little.
Count the busiest turnover day
Start with the worst day the calendar can produce, not the average month. Write down the most people who could be in the house on one day, how many showers and toilet flushes that group is likely to add, and how many loads of laundry and dishwasher cycles will run between stays. A house that hosts a group of eight on a weekend with a same-day turnover is a different problem from a house that hosts two people at a time, even if both have four bedrooms.
Ask the seller or installer to base the quote on that busiest day. If the quote is built on a family of four using water every day, the unit may suit a quiet month and be too small for the weekends that bring in the most income.
Check the regeneration timing against check-in
A softener periodically recharges its resin, and that recharge uses water. If it runs during a check-in window, guests may notice a pause or a drop in flow at the tap. Ask the installer when regeneration is set to run, and whether the start time can move to an hour when the house is normally empty. Ask how the controller handles a day with far more water use than usual, and whether it can be set to recharge sooner during busy stretches. Get the answers in writing so whoever manages the property later knows what the settings are.
Also ask what happens during long empty stretches, such as an off-season month with no bookings. A system that sits unused for weeks needs a plan, and the plan should be written down before the house goes quiet. The Missouri Water Softener Buyer's Guide walks through the wider sequence of choices.
Compare quotes on the same numbers
Give every installer the same inputs: the hardness reading from a test of the water entering the house, the number of bathrooms, the expected peak occupancy, the laundry setup, and the water heater size. Quotes built on different inputs cannot be compared fairly. Then read the rated capacity and flow the unit is given, and check that your busiest-day estimate fits inside it with room to spare. If an installer cannot explain how the unit was matched to the peak day, ask that question before any deposit is paid.
Also ask who handles salt refills, who responds when a guest reports low pressure or a salty taste, and how soon a service visit can be booked. The methodology page explains how this site assesses providers, so you can see which criteria were used when comparing them.