For the moment, there are three factory magazines available for the FN 510 pistol:
The FN 510 15-round magazine is excellent if you need a no-frills, flush-fitting magazine.
However, if you want the most capacity you can get, then you would love the FN 510 22-round extended magazine.
Last, if you are restricted to a 10-round capacity, FN makes a factory FN 510 10-round magazine.