As a 1 man band, you have zero chance of doing 20 cars per month, there is simply not enough man hours in a day, especially if you do the job properly:
MOT
Service
Cosmetic Repairs
Valet
All pre sale.
I am on for 12 this month between 2 of us and to achieve 12, we are entertaining 30+ visitors in the month (only work 5 days per week). On Friday, we had 1 customers pop back for a tiny job, 4 walk-ins, 1 appointment, 2 cars for MOT and another for Service plus 3 cars to prep, photograph and advertise.