In creatures supposits are always composite. So, for example:
This matter is not one individual except with reference to its substantial form. Matter as having quantity is in potency to being multiple individuals. A bird dies and rots and its matter is taken up by maggots and worms.
This form is not one except as individuated in this matter. Forms are always universal and thus potentially plural, able to inhere in multiple things.
This essence is not one except as actualized by this esse. There are no non-existent individuals.
This esse is not one except as terminated in this essence. The act of being considered as actually separate from any particular quiddity is incoherent except in the case of God. What is? The individual must have an essence.