Around 70 million of these are permanently settled systems (~7%), represented in some fashion in the Galactic Empire (or the Republic before it).
Before it was disbanded, the Galactic Senate had around 12,000 senators; each representing a sector of space including hundreds or thousands of worlds. Over 1 million full member worlds, supplemented by colonies, protectorates and governorships, brought the total number of member or member-aligned worlds to 70 million.
100 quadrillion beings live in the galaxy (one thousand million million!). This includes 20 million different intelligent species.
A quote from Star Wars Technical Commentaries: Astrophysical Concerns, theforce.net:
Each sector consists of several hundreds or thousands of inhabited systems. However the fraction of all stars actually possessing a hospitable world is so low, approximately one in ten thousand, that a typical sector will contain millions of barren stars as well. The typical separation of permanently settled systems must be dozens to hundreds of light years. The night sky seen from the surface of a typical inhabited world shows only about two thousand stars visible to the naked human eye. In most parts of the galaxy a planet must be considered fortunate if there is another habitable star system near enough to appear as even a mere point of light to the casual stargazer.
Another quote from Star Wars Technical Commentaries: Astrophysical Concerns, theforce.net:
Among the hundreds of billions of stars in the galaxy, most systems are uninhabited. The barren systems are still rich in mineral and other natural resources. These are what supports the galaxy's immense population. A civilisation existing on a single homeworld can only sustain a population of several billion human-like creatures at once, due to environmental constraints. With the resources of millions of barren worlds supporting each inhabited planet of the Galactic Empire, planetary populations could comfortably grow to trillions on any particular world. Raw materials and foodstuffs could be imported in arbitrarily vast amounts, and the empty wastes of space and dead worlds allow for indefinite waste disposal.
The entire duration of recorded history is ~500,000 years.
At the height of its power, the Imperial Navy had some 25,000 star destroyers at its command.
The Imperial Navy only rarely uses the rank Grand Admiral. There are less than 5 beings of such rank at this time.
The Galactic Empire has some 20 Oversectors, also called Priority Sectors (when an active military campaign or project was underway). Each of these is led by a Grand Moff.
Imperial space is divided into 1024 sectors, each headed by a Moff.
In the navy there are some 2000 High Admirals, most scattered across the galaxy heading Sector Fleets. Many are assigned to Naval Headquarters on Coruscant, as well. A similar number of Imperial Army Surface Marshals exist, with most of them commanding the entirety of a given sector's ground forces.
Around 8000 Fleet Admirals, 10,000 Admirals, 15,000 Vice-Admirals, and 35,000 Rear-Admirals round out the Navy's flag ranks.
Star Wars The Essential Atlas, LucasBooks, 2009 (Wallace, Fry)
Star Wars Technical Commentaries: Astrophysical Concerns, theforce.net, 2000 (Saxton)