Why is the carbon dioxide concentration a limiting factor of the photosynthesis process? When the carbon dioxide concentration is increased indefinitely is photosynthesis also increased indefinitely?

The availability of carbon dioxide is a limiting factor for the photosynthesis process because this gas is a reagent of the reaction.

As enzymes catalyze the building of organic molecules with carbon atoms from carbon dioxide photosynthesis stops as soon as these enzymes become saturated, i.e., when all their activation centers are bound to their substrates. In that situation an enhance of the carbon dioxide concentration will not enhance the photosynthesis rate.