In Diamond and Shurman's book on modular forms, they define
Definition 1.2.1. A subgroup $\Gamma$ of $\text{SL}_2(\mathbf{Z})$ is a congruence subgroup if $\Gamma(N)\subset \Gamma$ for some $N\in\mathbf{Z}^+$, in which case $\Gamma$ is a congruence subgroup of level $N$.
Clearly, $\Gamma(N)\subset \text{SL}_2(\mathbf{Z})$ for every $N$. Does that make $\text{SL}_2(\mathbf{Z})$ a congruence subgroup of level $N$ for every $N$, or are we choosing a minimum $N$ for which the definition is satisfied, i.e. $\text{SL}_2(\mathbf{Z})$ is level $1$ only.