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