The standard plant enzyme Rubisco consists of 16 protein subunits: eight large subunits and eight small subunits.
Rubisco is short for ribulose-1,5-bisphosphate carboxylase/oxygenase. In plants, algae, and cyanobacteria, its common form is a hexadecamer with an L8S8 arrangement. The eight large subunits form the main catalytic core, while the eight small subunits surround and help stabilize the complex.
Each large subunit contributes a catalytic site at an interface between large-subunit pairs. Rubisco catalyzes the first major carbon-fixation step of the Calvin cycle, attaching carbon dioxide to ribulose-1,5-bisphosphate. The resulting compounds are eventually used to build sugars.
Rubisco is abundant because it works relatively slowly and can also react with oxygen, initiating photorespiration. The number 16 applies to the common Form I enzyme found in plants; other organisms possess alternative Rubisco forms with different arrangements, including forms that lack small subunits. That biological variation does not change the canonical answer for the plant enzyme in this question.