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If I want to change the way star systems generate so each star has an average of about 5 planets and can have as few as 2, how would I do this?
You could play with values of the Planetary System generator and see what you get. Open your main-user.cfg
(make a copy first, in case you want to return to original values), you'll find it in your SpaceEngine \config folder, then search for this section:
// Planetary system generator probabilities
ProbBinaryStar 0.5 // binary system probability - 50% of all systems
ProbTripleStar 0.11 // triple system probability - 11% of binary systems
ProbQuadrupleStar 0.22 // quadruple system probability - 22% of triple systems
ProbWolfRayetStar 0.05 // probability of Wolf-Rayet star among all O-class stars
ProbCarbonStar 0.08 // probability of Carbon/Zirconium star among all red giant stars
ProbPlanemo 0.45 // probability of planemo (rogue planet) among all systems at the last octree level
ProbSolitaryWD 0.033 // probability of solitary white dwarf among all systems at the last octree level
ProbSolitaryNS 0.00025 // probability of solitary neutron star among all systems at the last octree level
ProbSolitaryBH 3e-005 // probability of solitary black hole among all systems at the last octree level
ProbCompanionWD 0.01 // probability of companion white dwarf among all binary systems
ProbCompanionNS 0.005 // probability of companion neutron star among all binary systems
ProbCompanionBH 0.0075 // probability of companion black hole among all binary systems
ProbBrownDwarf 0.025 // brown dwarf probability
ProbGasGiant 0.36 // gas giant probability
ProbHotGasGiant 0.012 // hot gas giant probability
ProbDryOceania 0.1 // probability of turning oceania into terra
ProbBinaryPlanetMoons 0.4 // probability of outer moons around binary planet
You can tweak those values (they're probabilities, so values range from 0 to 1).
Be advised, though, that your SpaceEngine will cease to have the exact same planets than everybody else, so you'll not be able to share your findings with others.