"Across the executive branch, acting officers are currently serving in vacant advice and consent positions, temporarily filling these offices until a permanent officer is nominated by the President and confirmed by the Senate. Some of these offices remain vacant because the Senate is still considering whether to confirm a nominated officer, but others are vacant because President Trump has not yet submitted a nomination to the Senate. According to one tracker, there are currently 277 offices that have no formal nominee. Some of the acting officers filling these vacant advice and consent positions may soon face an important deadline--one that could limit their ability to continue performing the duties of the vacant office"
CRS Legal Sidebar, November 01, 2017
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html