RE: you only need one planet as a model for psychohistory
to create a theory, correct. In practise, who can say? Experiments can go wrong. I would say even one planet is one planet too many:- (do you like my dog's bollocks)
1. You only need one psychohistory as a model of this planet. Ouspensky's observation that history is the history of some crime or other, sums it up nicely (if not very nicely).
2. You only need one model as a psychohistory of the planet. Cue irony. Roll up folks, take away your preferred -ISM here - gratis, there's one for everybody, but one per customer only. (Trade-ins accepted). We're all chattering classes now, eligible for bread and circuses while the psychos are getting on with making history. (You don't seriously suppose they give a shit about a/the scientifically correct model?).
3. You only need one model as a planet of psychohistory. God knows! If you're going to have something like Evil/Satan/etc in your grand scheme of things, you only need the one. Any more than one is not greedy, it's just silly. (That's what I call ID).