honestly, we don't have and exact number. maybe another ten or so... all in all, Luffy and co. haven't visited that many canon islands.
There are four main islands in the Grand Line that one must pass through to reach the final destination of One Piece: Reverse Mountain, Water 7, Sabaody Archipelago, and Marineford. Traveling through these islands is a key part of the journey to find the legendary treasure.
We think it's on the last island in the Grand Line, Raftel.
It crosses through, and around, a number of islands and a lot of water in the Pacific Ocean. The line deviates from a straight line to keep it from passing through countries such as the USA (Alaska), or New Zealand, and island nation such as the Marshall Islands and the Aleutian Islands.
The international date line goes around all land, but the line of longitude 180° from the Prime Meridian passes through Chukotka, Russia and among the islands of Fiji.
Cheap cruises that sail through the Caribbean islands are Windstar Cruises, Freedom of the seas, Princess Cruise, Regent seven seas, Celebrity, and Norwegian Cruise line.
It is possible to ride a boat through the Grand Canyon, but reservations must be made over a year in advance AND the ride is not exactly a "cruise" ship.
Foss Islands Branch Line was created in 1880.
Play through the story line
Tawita Temoku is the Minister for the Line and Phoenix Islands Development for Kiribati.
Equator
Island Chain
Grand - CTA Red Line station - was created in 1943.
You have to play through the story line, at different stages it unlocks the different parts of the map.