NPCs do not influence how much money income facilities, or in your case, a garage makes. What does influence the amount of money - apart from the planetary stats - is the person you assign as operator of your garage. The higher that person's trading skill is, the more money you make, or the less money you lose.
Planet population and the number of jobs are used to determine the "Employment Rate" or ER, ER is a figure used in determining Facility Income or FI. For complete information here is a link to the FI rules.
NPCs don't work in facilities. You were already told that. The flats and jobs stats of facilities are relevant only to the statistics of the planet, and do not refer to individual NPCs owned by individuals or factions.
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."