Quick take
For one person with moderate hardness, a compact 24K–32K-class softener is often sufficient. Test hardness first and size from actual gallons × gpg rather than buying the smallest model automatically.
Estimate daily hardness load
If one person uses around 60–75 gallons per day and hardness is 10 gpg, daily hardness load is roughly 600–750 grains.
Seven-day example
At 750 grains per day, seven days equals only about 5,250 grains before reserve, well below the efficient working capacity of many 24K–32K systems.
Very hard water changes the result
At 25 gpg, the same 75 gallons creates about 1,875 grains per day. Capacity disappears much faster, so hardness matters more than headcount alone.
Avoid extreme oversizing
A huge softener in a low-use household can regenerate very infrequently and cost more without delivering meaningful benefit.