There are 94 locations in total through the whole of the map, visiting each location may take a while depending on what activities you are taking part in. Just progress through the game at your own progress and you should find most locations with little to no effort. You can use the following map to find all the locations you may be missing:
Map of all locations
(This map does not include hideout locations, see "Instinto Asesino" for the hideout location map)
Alternatively instead of using the above map you can keep track of your progress from your Rockstar Social profile if you have one. This profile may not update instantly so you may want to keep checking it.