I just double checked to make sure I was explaining it right. The icon at the top of the screen that looks like two people standing next to each other with arrows facing one another, click on that. Then the list of players in your server will be shown. Find your name in that list (alphabetical) and highlight your name, by clicking on it. Once you've done that, you should be able to click the "Details" button on the bottom middle-right of the screen. It should bring up a pop-up window and "Exploration" should be the second stat I believe. There it will tell you how many Sectors you have explored.