Quick take
A 48K-class unit may work with moderate hardness, but 64K or larger becomes more compelling as hardness, iron and peak-flow demand rise.
Daily water demand adds up quickly
At roughly 360–450 gallons per day, even moderate hardness creates a substantial daily grain load.
Example at 10 gpg
A six-person household using 420 gallons per day at 10 gpg consumes about 4,200 grains of capacity daily.
Example at 20 gpg
At 20 gpg, the same household consumes about 8,400 grains daily. A small softener would regenerate very frequently.
Service flow is critical
Large households can draw water from several fixtures at once. Compare published service flow, not just grain capacity, before choosing the system.