US
Australia
New Zealand
South Africa
Namibia
Spain
Myanmar
Thailand
China
Argentina
Brazil
Peru
Venezuela
I have barely travelled.
[ul]
[li]New Zealand[/li][li]Australia[/li][li]Fiji*[/li][/ul]
*Barely counts, we had to take a cross-country plane from Nadi to Suva to change planes to get home.
Four that I can recall off the top of my head:
USA
Kenya
Tanzania
Australia
Canada
US
Thailand
Indonesia
India
Peru
Columbia
Venezuela
[ul]
[li]US[/li][li]Canada[/li][li]Switzerland[/li][li]Thailand[/li][li]New Zealand[/li][li]Australia[/li][/ul]
USA and Guatemala.
US, the Bahamas, Australia, Chile, and (if you count connecting flights when the first leg of your trip originated in another country), Canada. I think that’s it.
I think only the US. Which is kinduv weird. I’ve flown from Toronto to various points in the USA, and to England, but never to any other place in Canada as far as I can work out. 
US and Ecuador
United States
Canada
Ecuador
Peru
Australia
France
Italy
Germany
Looks like Colibri is the winner so far with 19, with smithsb a close second at 15.
US
Germany
Egypt
Japan
– US
– Canada
– Spain
– UK
– Germany
– Japan
– India
– Brazil
I was going to add Switzerland as I have flown in and out of Zurich a number of times, but only directly into and out of Switzerland. When the OP says domestically, I am taking this as meaning from one airport in the country to another in the same country. OP, please clarify.
I have also flown many times into and out of CDG but never as departure to or destination from another airport in France, so I didn’t include France in my list. Panama would also have made my list if I’d flown point-to-point within the country instead of solely in and out of the country via Tocumen. Same with Republica Dominicana and Argentina.
Yes, the flight must begin and end inside the same country. I’ve flown to and from lots of countries but only inside six of them from point to point. For example, I’ve flown to and from Japan, but I can’t count Japan because inside the country I just took trains.
Good. Thanks. Then my list is accurate.
UK
US
Canada
New Zealand
Norway
Two- the US and UK. On my first trip, we flew into Gatwick, and then got on some little puddle-jumper and flew to Aberdeen. About a week later, we flew back to Gatwick.
USA, Peru
UK (way more times than I can count)
Spain (Mallorca to Barcelona)
USA (Newark to Tampa)
Argentina (Mendoza to Buenos Aires; Buenos Aires to Iguassu)
Brazil (Iguacu to Rio)
Kenya (Masai Mara to Nairobi)
Vietnam (Hanoi to Da Nang; Da Nang to Ho Chi Minh City)
Cuba (Havana to Holguin)
Peru (Lima to Cusco; Cusco to Arequipa; Arequipa to Puno; Puno back to Lima plus sightseeing flight over Nazca lines)
Egypt (Cairo to Aswan; Aswan to Abu Simbel and back; Luxor to Cairo)
More than I thought when I started typing, but still only ten.
Guatemala
Indonesia
Australia
Malaysia
Canada
Mexico
China
Egypt
USA