According to Gul'dan and other Legion minions, at least 10.000 worlds were destroyed/taken over by the Legion. And as shown in the skybox of Telogrus Rift and Star Augur fight, other worlds have fallen to the Old Gods.
So No, the number of available planets to conquer has been reduced drastically.
The whole point of the Legion was to destroy all worlds and they did a good job with it, so the Void Lords will have far more chances of hitting Azeroth than they did in Pre-Legion days.
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There's no evidence of this. Azeroth is a big planet and clearly there are still parts of the planet that are unknown/undiscovered, like the part where Avaloren is located, or the land where the Titan facility "Uldaz" was built.
The only satellite orbiting Azeroth is the Vindicaar and it observes only the side of Azeroth where Kalimdor and EK are located. So the Vindicaar will not detect what is happening on other side of Azeroth.