So far I've seen systems with 7 planets.Planet quantity is lower now that there are solar systems and planets with defined "orbits"
There used to be dozens of planets per system, now there's around 4, max.
Systems occasionally have 2 planets in an orbit instead of one, and there's still the rare free floating planet. But 1-4 is a good general number for the amount in a system.So far I've seen systems with 7 planets.
I don't think there's anything in the server's config that effects the number of planets; however you can increase the sector size (so there's the same number of planets per system, but the system is much larger and those planets are further apart).