An older house can have a softener sized correctly for the people living there and still starve the fixtures on the far side of the home. Headcount is only half of the sizing question. The other half is how much water the pipes feeding the softener can deliver, and how many fixtures draw at the same time.
Start with the pipe, not the bedroom count
Find where the main water line enters the house and note its diameter and material. Older homes may have galvanized steel runs or older copper with narrower bores than modern fittings, and the pipe near the meter may be smaller than the connections at the softener location. A narrow line caps how much water reaches the unit, whatever the label on the softener says.
Read the flow rating, not only the capacity
Softener specifications list two numbers that matter for fit. One is how much hardness the unit can remove between regenerations. The other is how much water per minute it can pass at a usable pressure. Buyers tend to compare the first and skip the second. In a house with a small supply line, a unit with strong capacity but a low flow rating can drop pressure at the shower when two fixtures run together. Ask for the flow rating at the pressure your house actually has, and ask the company to put it in writing.
Count the fixtures that run together
Walk the house and list every fixture that can run at once: showers, the washer, the dishwasher, an outdoor spigot, an irrigation line if it connects to the house supply, and any tub with a fill line. Then ask which two or three of them are likely to run during a busy morning. That tells you whether the softener is sized for simultaneous draw, which is where older houses usually run into trouble, not for an average day.
Ask the installer what they measured
A careful installer will check static pressure at the proposed location, look at the pipe size where it enters the house, and say plainly whether the existing run is a bottleneck. If the only inputs were the bedroom count and the meter reading, that was not a sizing check. Compare the written sizing notes from two or three companies. Our best water softener companies in St. Louis list is a place to start, but the sizing questions below work with any company you consider.
Write down five points before you compare quotes
Before you accept a proposal, make sure these are written down: the main line size and material, the measured pressure, the rated flow at that pressure, the fixtures that run at once, and whether the installer expects any pipe work. A quote that skips any of the five is a quote for a different house.