Star Trucker

There are a total of 32 sectors. Initially you'll only have access to a few, as you do more side missions you'll unlock more areas.
Just visit them once to count, there is a counter in the bottom right of the map that says a % of how many sectors you've visited.
I would recommend as you unlock each new area visit each sector so you don't miss any.
The final area is unlocked during the final mission for G-Bee