Quick take
Start with 32K for moderate hardness, then move upward if hardness is high, iron is present or the calculated regeneration interval becomes too short.
Example at 10 gpg
Two people using roughly 120–150 gallons per day create a daily hardness load of about 1,200–1,500 grains at 10 gpg.
Example at 20 gpg
At 20 gpg, that same water use creates 2,400–3,000 grains per day. Seven days can consume roughly 16,800–21,000 grains.
Why 32K can still work
Many 32K-class systems have enough usable capacity for a two-person household when set efficiently, especially at moderate hardness.
When to choose 48K
Move to 48K when hardness is very high, water use is high, iron adds load or you want a longer interval between regenerations.