You cannot do two shops at the same store on the same day. All of the shops must be at least 3 days apart. There should be only a total of 4 shops per location, 2 full service and 2 self-service. I don't know if they limit your ability to do all 4 of them in the same round.
I don't remember if the shop limit per day, over multiple locations, is 4 or 5, I mostly only do a max of 3 per day until it's a long distance route, then I will do 4 to avoid having to do the long drive again. What Dee said about the 15 shops at the same location per year is correct, and their calendar year resets usually in February.