Your water bill will specify all water consumed which will exceed the soft water utilized by your family. Additional water that will be consumed will include that utilized for irrigation, and the water the softener utilizes for regeneration.
Greater softener capacity will typically result in less water needed per month/year for regeneration. To obtain the best balance of salt efficiency, soft water quality & usable capacity (lower frequency for regeneration), the usual recommended settings will be 24,000 grains usable capacity per cubic foot of resin which will require 8 lbs salt per cubic foot to regenerate.
Using these settings, a 1.5 ft3 softener (48K grains Total Capacity) will provide up to 36,000 grains usable capacity (36,000 / 19 GPG = 1,894 gallons) while consuming 12 lbs salt per cycle.
A softener containing 2 ft3 resin (64K grains Total Capacity) will provide up to 48,000 grains usable capacity (2,526 gallons) while using 16 lbs salt each cycle.
Since your water source is municipal which will be chlorinated and will therefore contain no iron for the softener to remove, the usual recommended regeneration frequency is > 7-days, but less than 30-days. Even if 12,000 gallons is consistently consumed per quarter (3,000 gallons/month = 100 gallons/day), a 2ft3 unit will likely need to regenerate every 24-25 days which is perfectly OK.