Let E be an elliptic curve without complex multiplication defined over the rationals. The purpose of this talk is to define a positive integer A(E), that we call the Serre’s constant associated to E, that gives necessary conditions to conclude that $ρ_{E,m}$ , the mod m Galois representation associated to E, is non-surjective. In particular, if there exists a prime factor p of m satisfying $val_p (m) > val_p (A(E))$ then $ρ_{E,m}$ is non-surjective. We determine all the Serre’s constants of elliptic curves without complex multiplication over the rationals that occur infinitely often. Moreover, we give all the possible combination of mod p Galois representations that occur for infinitely many non-isomorphic classes of non-CM elliptic curves over $\mathbb{Q}$, and the known cases that appear only finitely. We obtain similar results for the possible combination of maximal nonsurjective subgroups of $GL_2 (\mathbb{Z}_p )$. Finally, we conjecture all the possibilities of these combinations and in particular all the possibilities of these Serre’s constant. This is joint work with Harris B. Daniels.