Extremal combinatorics is the study of the size that a collection of objects must have in order to certainly satisfy a given property. Reaction systems are a recent formalism for computation inspired by chemical reactions. This work is a first contribution to the study of the behavior of large reaction systems by means of extremal combinatorics. We define several different properties that capture some basic and dynamical behaviors of a reaction system and we prove that they must necessarily be satisfied if the system is large enough. Explicit bounds and formulae are also provided.