Don't forget Jersey and Guernsey!
IOM and Gibraltar would count as seperate counties. Don't forget there are a few islands scattered around the world that form part or are dependent on another larger nation, and you can't really count them as France, USA or UK (Guadeloupe, Aruba, Puerto Rico, Cayman Islands, BVI, Bermuda, Tahiti, Cook Islands, Easter Island, Greenland, Falklands etc) and of course there is Antarctica and what about Hong Kong?
Being from Jersey, visiting the UK countries, Guernsey and Alderney tend to be for week long holidays, so therefore count towards my total. (same goes for Madeira)
That brings my total to a paltry 15.
But a number of them I have visited multiple times, only to different regions, and will be returning to two of them for my second trip to Greece (Corinth Coast) next month (but the first time was a morning port call in Athens,) and to Sicily in September the 5th part of Italy i have been to. China would probably be my 16th destination, would Hong Kong count as number 17?