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

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

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