Ok, Im guessing wearing once refers to dressy shirts that you wear a tie and tuck into your pants with.
The question I have is how many times do you wear normal button up shirts before washing. You know the kind that isn't meant to be tucked in usually worn casually with thicker material.
THIS I have the definitive answer for. ONCE! Buy only non-iron natural material shirts and do the collar & cuff thing mentioned above, put it on a hanger & when you have 3-5 of them dirty wash & then hang dry. Sometimes life is simple.