On the one hand, Thomistic theory explains the multiplication of individuals through matter. But another element in individuation is the consequent limitation of form by matter. Form as such has to be visualized as broader and more expansive than the form of the composite, and as having in itself a fuller range of perfection than can be actualized in any composite with that form.
An example of this is personality types. The analysis of personality types seems to indicate a structure in the human species as such, where each of the types is a partial expression of what is a formal totality. No one individual can be both extraverted and introverted, but it’s hard for me not to see these as parts of a larger formal totality. Aristotle’s account of the social division of persons at the beginning of the Politics – even if incorrect – points to the same thing. Most fundamentally, the division of male and female in living things bespeaks an intrinsic complementary existence in life itself, or at least in sexually reproductive life.
More briefly, and even if my above examples are wrong, it is contrary to the very nature of species that there should be one individual expressing all possible perfections of that species. Even though multiplication of species happens by matter, it is intrinsically necessary to the perfection of natural species, and so of the form, that there be a multitude of members.
Now it is not necessary to form as such that it have the this sort of intrinsic complementarity and/or hierarchy that cannot be instantiated except in a multitude. In the measure that form subsists of itself, then the self exists for itself and not as a part of some greater formal totality. Human beings attain this subsistence in perhaps the faintest possible way, and it is always conditioned by the latticework of complementarity and partial formal existence. Our perspective is always male or female, introverted or extraverted, young or middle aged or old, etc. Nevertheless, in the measure that we are rational there is always the suggestion, perhaps even the hope, of transcending this perspective. This is one way of understanding the way in which man is a subsistent form – it is not that there is one and only one man, but that human rationality always brings with it the suggestion of transcending human nature by a purely unlimited self – for better and for worse.