There is a longstanding read-back of neo-Platonist monadism into Plato's own Form of the Good (though see also William Altman, "Why Plato's Idea of the Good Is Not the One"). Now for Plato (either way), the One leads to the Two, the Two to the Three, and so on forever. There is also a Large Form/Form of the Large, which lends itself to the ordering on the Numbers, presumably (though for where Plato speaks of the Good being greater in glory and might than the other Forms, one wonders how It would be so if Its glory and might did not participate in the Form of the Large? whence is It not the Largest Number of all instead?).
Anyway, the great difference between the Forms and the Earthly exemplars is that the Forms are perfect self-participants. Translated: they are otherwise as general to their terms as possible, not saturated with the differentiae that adulterate the essences of the lower exemplars. Auto to dikaiosune is not just on account of the differentiae that would also shape it in other directions. If there were only one possible exemplar of a Form, i.e. that Form Itself, the reason for distinguishing between general and particular truths would disappear. (C.f. why the most jealous of all possible deities is not held to be "divided into" a general essence and a particular expression of that essence.)
But though the Two-in-itself might be construed as a Form of the Two (or of Duality, or whatever), for elsewhere Plato writes of Forms of anything and everything (over any and every descriptive term), just the same, it is the multiplicity that is their essence that makes the Numbers into an ambient source for the possibility of imperfection. If there had been no Numbers, nothing would have been able to differentiate itself from a pure self-exemplar so as to make counting pluralities of exemplars possible. Conceivably even the other Forms would never have been parted at all from that of the Good.
Whether Plato thought strictly as such, or rather recognized such apparent implications of some claims he floats (if not outright makes), I don't know, but I suspect not quite. Still, he has no clear-eyed appreciation for the problem of evil's full scale in his own transtheology (if he was ever even so dogmatic as to think of himself as trying to advance a settled transtheology!), which problem infects his known discourse on Forms, participation, self-exemplification, demiurges and world-souls, and so on. That is, it seems as if Plato probably has to locate the source of evil, as participatory imperfection, in the Forms somehow, yet then the Forms with this unsavory role in reality then get depreciated along the line of knowledge in The Republic (as interfering with perfect knowledge, as united knowledge, by virtue(!) of their multiplicity, which divides the intrinsic Form of Knowledge itself no less?).
