If you tally up all the locations that can be discovered on the map you get with the strategy guide book they come to 531.
I just did a quick count in the CK, and there are
397 map markers defined in the vanilla Skyrim data set.
Now, some of them will only show up if you go through certain quests, mainly the war campaign but one should be able to unlock all of the 397.
WARNING, SPOILER ALERT:If you are interested, i uploaded a XLS file on my server with a list and the individual cell coordinates.
http://sirandy.org/skyrim/MapMarkerData.xlsx
