Early means ‘fast to produce’, at any time of year, and often means fast to flower too! I would sow a few of each to have more chance of them not finishing at the same time.
For 3 people, regular lettuce could come from 20 or 30 plants, hard to be precise because of variations in weather, the size you pick them (pick large for higher yield), varieties, soil quality and amount needed. 20-24 should be enough.
