Only a limited number of planets can be directly controlled before suffering inefficiency penalties to  Influence as well as empire-wide  income. At this point, it is best to start delegating excess planets to Sectors. A Sector is a semi-independent administrative region under the control of a Governor. It will govern itself, though the empire remains responsible for the defense and protection of its sectors and can issue some priorities and orders to the sector government.
Each sector has its own stock of  energy and  minerals. These stocks will be filled with sources within the sector's borders, except that some of its income may be sent to the national stockpile depending on what tax level is selected for it. The sector's stocks can also be boosted in 100 unit increments from the national stockpile.  Science and  influence income go to the empire regardless of tax.  Strategic resources can be used by a sector only if the sector itself has them; any surplus goes to the empire, and sectors cannot use resources from other sectors or core worlds. Since sectors do not have their own Influence, frontier outposts controlled by them do not cost Influence to maintain.
Sectors remain an integrated part of the empire, but handle development of planets and the construction of stations and defensive armies within their region themselves. What exactly they build is influenced by the focus set for them: military (focusing on defence stations), industrial (focusing on minerals), scientific, or economic (focusing on energy). They can be permitted or forbidden to replace existing buildings and/or requested to respect existing resource deposits. Sectors do not possess or build any military fleets or offensive armies of their own, so the empire is responsible for their defense and protection.
The exact limit for direct control of planets depends on various factors, such as government type and technologies. Sectors are created by clicking on the empire tab at the top left of the screen. Sectors can be created and removed at will and assigned a name and sector capital of the player's choice. Star systems can be added to and exchanged between sectors at any time, but each sector must always be a single contiguous region of connected space.
The technologies to increase the maximum number core worlds and sectors are both techs that may be researched repeatedly, although at increasing cost.
Removing star systems from a sector costs  25 and removing sectors costs  100. However, if the sector is smaller than 4 star systems, it costs less influence to remove each star system manually until the sector is removed.