Surprisingly, at least in some circumstances, just one. It is not clear, however, that he or she will know when to change the tire--or get the "details" right.
Today, I got a bit worried when my front right tire scratched against a cement barrier after a sharp turn, so shortly afterward, I stopped at a Staples parking lot to check out the tire. Sure enough, it looked like it might have lost some air. Luck had it that Costco, where I had been headed, was only 0.4 miles away, so I changed the tire to the spare and brought it into the tire shop at Costco. The technician who looked at it did not see anything wrong, and measured that the tire actually had a normal 26 lbs. pressure. I showed him a bit of an indentation on the side of the tire, but he said that this would not be a problem. I was concerned about how sagging the tire had seemed, but he then explained to me that front tires "always look flat" because they carry "2,000 more lbs. of weight" than the back tires. So, I ended up changing back from the spare to the "original."
Back many years ago, a tire technician laughed when he was about to put the tire he had fixed back on and noticed that I had turned the wheel the wrong way.
This is clearly not my area of competence!