Count screens, not people
The number that matters is how many screens could be running at the same time on your busiest evening, not how many people sleep in the house. A couple who watch together need one. A house where the kids have a bedroom TV and the football is on downstairs needs two, and a third if somebody is also propped up in the kitchen with a tablet during those same hours. Three screens at once means three lines, and three lines means three times the cost, so be realistic rather than generous with this number.