Ann and Becky have worked for 8 hours each, so both of them worked for 2 people * 8 hours/person = <<2*8=16>>16 hours.
All three of them, including Julia, were working for 16 hours + 6 hours = 22 hours.
So in total, they all have served 7 customers/hour * 22 hours = <<7*22=154>>154 customers in one day.
#### 154