In which type of chemical reaction due to a more reactants combined to form one product only

Synthesis or a combination reaction. Two of the reactants form one final product in the end.

As we know that a type of chemical reaction in which two elements as reactants:

C(s) + O2(g) ∆> CO2(g)

Here, ∆ = Heat Energy.

Here, reactants = C(s) + O2(g)

and, product = CO2(g)

This type of reaction is known as combination reaction. It is also known as addition reaction.