How much real estate is out there?

Short answer: A lot.

Long answer:

In the Solar System, there are eight planets, several large moons besides Ganymede, and several spherical asteroids besides Ceres.

Outside the Solar System, astronomers discover new planets every month. There are over one thousand confirmed extrasolar planets (exo-planets). If you add in Kepler candidates, then the total is above five thousand.

Alan Boss, astrophysicist at the Carnegie Institution in Washington, D.C., estimates at least one quintillion (1,000,000,000,000,000,000) planets in the Milky Way plus all other galaxies in the universe.

Fortunately, many physicists think that there are other universes in the multiverse; so do not worry about running out of planets. Some may not even need terraforming.